The BaseCollector and BaseRenderer are merged into
the BaseHandler (as mixins for now).
Developers are still able to create collectors
and renderers using these deprecated base classes,
and pass instances of them when creating their handler.

All the methods of the deprecated bases can now
be defined on the BaseHandler subclass itself.
Handlers can then be instantiated by passing
the handler's name, the theme and the optional
custom templates folder name/path.

Reasoning: often times, the renderer and the collector
need to communicate or share data. For example,
the Crystal renderer uses the collector to lookup names
when creating cross-references. The Python handlers
are able to filter members when collecting/returning data,
and need the same members list when rendering,
to order the elements based on that list.

This change is the first of two, where the second change
will deprecate the use of `selection` and `rendering` keys
in the YAML options or MkDocs configuration, in favor of
a single `options` key that both the collection and rendering
process will share.

Co-authored-by: Oleh Prypin <oleh@pryp.in>
Issue #418: #418
PR #419: #419

As seen in commit eb822cb, the separation of
the selection/collection and rendering configuration
is problematic. This commit further reduces this
separation by merging the two current YAML keys
'selection' and 'rendering' into a single
'options' key. It means both the 'collect' and
'render' methods have access to every option.

The old keys can still be used, but are deprecated,
and therefore a deprecation warning is emitted
if these keys are found in either global or local
configurations.

PR #420: #420

This is to allow handlers to build potential paths
(like Python search paths) relative to the MkDocs
configuration file path instead of the current
working directory. Without it, the build becomes
dependent on the CWD, breaking the --config-file
option.

Issue #311: #311
PR #425: #425