Now we try to harvest as much info as possible from the function
metadata (i.e. args, kwargs, default values) and function
docstrings, assuming that the markup is sphinxy python (i.e.,
:param foo: help for foo).  This saves a ton of redundant typing in the
decorators.  The ApplicableMethod decorator can be told *not* to auto-
populate from the metadata and docstrings, *or* it can be told to
exclude certain function parameters from promotion to argparse
parameters.

Also, drink our own koolaid by creating ApplicableFormatters,
so we can have custom formatters per-function, that each have their
own arguments.  To reuse all this argparse automation code, they have to
be decorators on the formatter function.