To be in sync with the new header style used in docs.sencha.com.

Reimplement Guides#each_item so that the guide doesn't get passed
to the block when loading of it had failed.

Also make Logger#warn return false to be able to return with false
right after showing warning.

To allow easier changing of title of the produced HTML file.

The compare script now lists added, removed and changed members.

The change tracking is currently targeted towards configs. We
detect the following changes:  type, default value, visibility.

I refactored the script so it now finally has a sensible interface,
with options --ignore (for the optional ignored methods/classes file)
and --type (to limit output to just configs).

Use class and member names instead of indexes.

When documentation contains @since and/or @new tags, they always
take precedence over auto-generated ones.

Along with @since tags generate @new tags.

@new tag is added to any class/member that is available since the
very latest version.

Graphically the @new tags will be rendered as stars on yellow
background.

Consequently OldVersions module now becomes Importer.

Newer connect-mongo versions cause trouble. Needs investigation.

The new --old-version option allows one to point JSDuck at doc exports
for different versions.  JSDuck will then import the exported data and
generate @since tags based on that.

For start only classes get @since tags.

The Class#get_members now allows three values for the 'static'
parameter: true, false, nil.  False can now be used to grab only
instance members, while nil will return both instance & static.

When expanding widget.* enums, set enum properties as private
when the class defining the alias is private.

It was at the wrong level - foo.* enums were reported
as empty.

So one can declare:

    @enum [xtypes=widget.*]

and it will be automatically populated with all available widget types.

Auto detected properties default to being private normally, but for
enums, all possible values should be public.

    /** @enum {String} */
    Foo = {foo: 1, bar: 2, baz: 3};

It acts just like class with special enum=true flag.

The values of an enum are just class properties.

This will automatically choose an available Markdown implementation.

Additionally added missing require statement to doc_formatter_spec.

A regression. This worked in JSDuck 3.x but the behavior was lost
during parser updates.  Now additionally covered with unit tests.

This premature optimization was causing a strange test failure
regarding MetaTagRegistry.  I did some speed measurements and
there seemed to be no performance benefit of having this class
being singleton.