Support terminal coloring in Vim 8

Vim 8 supports the new :terminal command. Zenburn had no color support
for it yet, so add it.

Note: the system colors (set e.g. via LS_COLORS in Linux) will be shown
in the terminal window. The highlight setting only overrides the
"default" color.

Fixes jnurmine#21.

Signed-off-by: Jani Nurminen <[email protected]>