minirst: support containers
Text can be grouped into generic containers in reStructuredText:
.. container:: foo
This is text inside a "foo" container.
.. container:: bar
This is nested inside two containers.
The minirst parser now recognizes these containers. The containers are
either pruned completely from the output (included all nested blocks)
or they are simply un-indented. So if 'foo' and 'bar' containers are
kept, the above example will result in:
This is text inside a "foo" container.
This is nested inside two containers.
If only 'foo' containers are kept, we get:
This is text inside a "foo" container.
No output is made if only 'bar' containers are kept.
This feature will come in handy for implementing different levels of
help output (e.g., verbose and debug level help texts).