Improve `Facebook\HHAPIDoc\MarkdownExt\AutoLinkifyFilter`

[fredemmott]

This filters code spans to automatically turn types into links - for example:

`MyClass` already becomes a link to "MyClass"'s documentation page
`MyClass<MyOtherClass>` should link to both `MyClass` and `MyOtherClass`'s documentation pages
`"foo": MyClass` should link to `MyClass`'s documentation page