Extract Js::ExtDefine class from TagRegistry.

TagRegistry doesn't need to deal with this stuff that's only used
in one place.  The Ext.define configs are only needed in Js::Class
and nowhere else.
Move CSS processing from TagRegistry to Web:Css.

Another responsibility that doesn't belong to TagRegistry.
Move html_renderers from TagRegistry to Render::Tags.

Again, not generic enough functionality to keep in TagRegistry.
Extract MemberRegistry from TagRegistry.

Yet another thing that's not really a concern for the TagRegistry,
as it only effects a small subset of tags.

As a result, the TagRegistry class is now quite clean.
Improve rake task docs.
Allow comma-separated list of paths for --exclude.

Refs #216
Allow comma-separated list of paths for --images.

So it behaves the same as other options that accept multiple paths.
Allow comma-separated paths for --tags.

Make this options also in line with other multiple-path-accepting
options.