Unify naming of TagRegistry methods.

Have two types of accessors:

 - get* methods for accessing one tag by some kind of key.
 - <plural noun> methods for accessing arrays of tags.
Extract TagLoader class from TagRegistry.
Reintroduce loading of custom tags.

The old --meta-tags options is now replaced with --tags, that
pretty much behaves the same, except the classes themselves
need to be implemented differently.
Move processing of @throws inside Throws tag class.
Reintroduce title for @new tag.
Extract Subproperties class from DocAst.