fix: honor special character replacements in charmap

This now returns the expected result with `_` included:

```
slug.charmap._ = '_'
slug('foo_bar baz') // 'foo_bar-baz' but was previously 'foobar-baz'
```

Fixes: Trott#198

slug.js

@@ -171,6 +171,8 @@
     // sort lengths in descending order.
     lengths = lengths.sort(function (a, b) { return b - a })
 
+    const disallowedChars = opts.mode === 'rfc3986' ? /[^\w\s\-.~]/ : /[^A-Za-z0-9\s]/
+
     let result = ''
     for (let char, i = 0, l = string.length; i < l; i++) {
       char = string[i]
@@ -190,14 +192,16 @@
           char = localeMap[char]
         } else if (opts.charmap[char]) {
           char = opts.charmap[char]
+        } else if (char.includes(opts.replacement)) {
+          // preserve the replacement character in case it is excluded by disallowedChars
+          char = char.replace(opts.replacement, ' ')
+        } else {
+          char = char.replace(disallowedChars, '')
         }
       }
-      // next line preserves the replacement character in case it is included in allowedChars
-      char = char.replace(opts.replacement, ' ')
       result += char
     }
-    const allowedChars = opts.mode === 'rfc3986' ? /[^\w\s\-.~]/g : /[^A-Za-z0-9\s]/g
-    result = result.replace(allowedChars, '') // allowed
+
     if (opts.remove) {
       result = result.replace(opts.remove, '')
     }

test/slug.test.js

@@ -82,6 +82,11 @@ describe('slug', function () {
     })
   })
 
+  it('should preserve punctuation added to charmap', function () {
+    slug.charmap._ = '_'
+    assert.strictEqual(slug('foo_bar baz'), 'foo_bar-baz')
+  })
+
   it('should replace latin chars', function () {
     const charMap = {
       À: 'A',