So the #parse method of Tag classes doesn't have to return this
empty field and maybe in the future we get rid of it altogether.

Also introduce shorthand for horizontal whitespace skipping.

This will then be added as a new tag through #add_tag.

Extract DocMap class for building map of @tags in comments.  Run
the map building just once inside FileParser - instead of repeating
this thing in DocType and DocAst.

No real speed advantage, mainly removal of duplication.

Instead of performing the map building in every #create_* method,
the map is passed as parameter to all of them.

The tagnames from doc-comments and final field name are now the same -
:aliases.  Instead of the former case when one was named :alias and the
other :aliases.

Create ClassListTag base class for processing four tags:

- @mixins
- @alternateClassName
- @requires
- @uses

For this to work out correctly, the application of default values
in merger needed to be changes so that defaults get applied even
when the field doesn't exist in neither docs or code hash.

That is for tags: @cfg, @property, @method, @event, @css_mixin,
@css_var.  For the latter introduced also a Tag class (although
there is no corresponding @tag).

The regex for matching member types now gets constructed in TagRegistry
class, which has the central knowledge of the available member types.
Similarly exporting uses TagRegistry.member_types to loop over all
available member types (removing hardcoded member types list from
JsDuck::Class#each_member_type).

Otherwise the documentation inheriting process will run into infinite
loop.

For example the following code cause a problem:

    /**
     * @class
     * @extends B
     */
    A = {};

    /**
     * @class
     * @extends A
     */
    B = {c: 'd'};

That's fixed now.

No more does using this option restrict viewing of inline examples
to WebKit based browsers.  This check is now moved over to
eg-iframe.html in Sencha Touch codebase.

This fixes the issue of return value docs being completely missing
in case @return wasn't followed by {TypeDef}.

Instead of using hardcoded image with text "Sencha Docs".

Don't direct people right away to JSDuck issue tracker - most likely
it's just a fault of a broken link and it's the authors of the
documentation who should fix it.

A silly mistake.

Allows for ignoring additional HTML tags which otherwise would get
closed down by the new automatic tag closing mechanism.

Useful for ignoring ExtJS preprocessor directives: <debug>, <locale>.

Fixes an old problem with some unclosed <b> or <div> messing up
the layout of docs.

In addition to automatically closing the tags, a warning gets printed.