Add regex for title text version of links

* Add regex for title text version of links

Original parser matched:
  [name](http:://link)

Add match for:
  [name](http:://link "title text")

* Add tests and improve regex's
* Several new tests in test_maddy_linkparser.cpp  (Some with paths for future improvement, and one to ensure an overzealous future update doesn't disallow actually-used special characters like o-umlaut).
* URLs now ignore leading/trailing spaces.
* URLs now don't match on internal spaces or quotes.
* Small grammar fix in CONTRIBUTING.md
* Updated changelog.

docs/definitions.md

@@ -41,6 +41,14 @@ results in
 <a href="http://example.com">Text of the link</a>
 ```
 
+```
+[Text of the link](http://example.com "title text")
+```
+results in
+```html
+<a href="http://example.com" title="title text">Text of the link</a>
+```
+
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