fix(mentions): allow for usernames with dot, underscore and dash

Closes #574

src/subParsers/anchors.js

@@ -75,7 +75,7 @@ showdown.subParser('anchors', function (text, options, globals) {
 
   // Lastly handle GithubMentions if option is enabled
   if (options.ghMentions) {
-    text = text.replace(/(^|\s)(\\)?(@([a-z\d\-]+))(?=[.!?;,[\]()]|\s|$)/gmi, function (wm, st, escape, mentions, username) {
+    text = text.replace(/(^|\s)(\\)?(@([a-z\d]+(?:[a-z\d.-]+?[a-z\d]+)*))/gmi, function (wm, st, escape, mentions, username) {
       if (escape === '\\') {
         return st + mentions;
       }

test/features/ghMentions.html

@@ -1,3 +1,10 @@
 <p>hello <a href="https://github.com/tivie">@tivie</a> how are you?</p>
 <p>this email [email protected] is not parsed</p>
-<p>this @mentions is not parsed also</p>
+<p>this @mentions is not parsed</p>
+<p><a href="https://github.com/john.doe">@john.doe</a></p>
+<p><a href="https://github.com/john-doe">@john-doe</a></p>
+<p>@.johndoe</p>
+<p>@_johndoe</p>
+<p>@-johndoe</p>
+<p><a href="https://github.com/johndoe">@johndoe</a>.</p>
+<p><a href="https://github.com/johndoe">@johndoe</a>-</p>

test/features/ghMentions.md

@@ -2,4 +2,18 @@ hello @tivie how are you?
 
 this email [email protected] is not parsed
 
-this \@mentions is not parsed also
+this \@mentions is not parsed
+
+@john.doe
+
+@john-doe
+
+@.johndoe
+
+@_johndoe
+
+@-johndoe
+
+@johndoe.
+
+@johndoe-