Add theme config for bolder vertical split line

Previously the `VertSplit` (1) (`:help VertSplit`) key used `nord1` as
background color by default making the line appear to be very lumpy.
This commit changes this style to use `nord0` as bavkground instead to
visually merge with the background so only the separator charaters are
a visual indiciator for the split line whih makes it look more
lightweight and unclutters the overall appeareance.

To allow user who liked the previous implementation to keep the style
this commit also adds a new `nord_bold_vertical_split_line` theme config
that can be assigned to `1` to achieve the legacy design.

The README also include information and hints how to change the
separator character by customizing Vim's `fillchars` (2)
(`:help fillchars`) variable.

References:
  (1) http://vimdoc.sourceforge.net/htmldoc/syntax.html#hl-VertSplit
  (2) http://vimdoc.sourceforge.net/htmldoc/options.html#'fillchars'

nordthemeGH-132

README.md

@@ -26,6 +26,7 @@ Nord Vim is a 16 colorspace theme build to run in GUI- and terminal mode with su
     - [Comment Contrast](#comment-contrast)
     - [Uniform diff Background](#uniform-diff-background)
     - [Line number background](#line-number-background)
+    - [Bold Vertical Split Line](#bold-vertical-split-line)
   - [Plugin Support](#plugin-support)
     - [UI Plugins](#ui-plugins)
     - [Language Plugins](#language-plugins)
@@ -259,6 +260,20 @@ let g:nord_cursor_line_number_background = 1
 <p align="center"><strong>No background (default)</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-config-cursor-line-number-background-default.png"/><br><strong>Enabled background</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-config-cursor-line-number-background.png"/>
 </p>
 
+### Bold Vertical Split Line
+
+Enables a bolder vertical split line by using `nord1` as background.
+
+```viml
+let g:nord_bold_vertical_split_line = 1
+```
+
+To change the separator character used to display the vertical line please see the documentation about Vim's [`fillchars`][vdoc-fillchars] variable: `:help fillchars`
+
+<p align="center"><strong>With base editor background (default)</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-config-bold-vertical-split-line-default.png"/></p>
+
+<p align="center"><strong>With enabled bold <code>nord1</code> background</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-config-bold-vertical-split-line.png"/></p>
+
 ## Plugin Support
 
 Nord Vim provides support for many third-party language- and UI plugins.
@@ -332,3 +347,4 @@ Please report issues/bugs, feature requests and suggestions for improvements to
 [scrot-lang-ruby]: https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-lang-ruby.png
 [scrot-readme-default-profile]: https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-readme-default-profile.png
 [scrot-readme-lazy-profile-change]: https://raw.githubusercontent.com/arcticicestudio/nord-vim/develop/assets/scrot-readme-lazy-profile-change.png
+[vdoc-fillchars]: http://vimdoc.sourceforge.net/htmldoc/options.html#'fillchars'

colors/nord.vim

@@ -122,6 +122,10 @@ if !exists("g:nord_cursor_line_number_background")
   let g:nord_cursor_line_number_background = 0
 endif
 
+if !exists("g:nord_bold_vertical_split_line")
+  let g:nord_bold_vertical_split_line = 0
+endif
+
 function! s:hi(group, guifg, guibg, ctermfg, ctermbg, attr, guisp)
   if a:guifg != ""
     exec "hi " . a:group . " guifg=" . a:guifg
@@ -248,7 +252,12 @@ call s:hi("TabLineSel", s:nord8_gui, s:nord3_gui, s:nord8_term, s:nord3_term, "N
 
 "+--- Window ---+
 call s:hi("Title", s:nord4_gui, "", "NONE", "", "NONE", "")
-call s:hi("VertSplit", s:nord2_gui, s:nord1_gui, s:nord3_term, s:nord1_term, "NONE", "")
+
+if g:nord_bold_vertical_split_line == 0
+  call s:hi("VertSplit", s:nord2_gui, s:nord0_gui, s:nord3_term, "NONE", "NONE", "")
+else
+  call s:hi("VertSplit", s:nord2_gui, s:nord1_gui, s:nord3_term, s:nord1_term, "NONE", "")
+endif
 
 "+----------------------+
 "+ Language Base Groups +