Allow for re-use and more independent testing.

'sfdoc' is easier to type and resembles 'pydoc' and 'perldoc' so it's
even easier to remember.

Great rewrite with revamped CLI, many added features and bugfixes.

 *  New CLI optimized for simple use.  Default action is to show
    module doc in manpage viewer.

 *  Respects variable documentation.

 *  Respects module documentation (first stray docstring).

 *  Can export (ugly but usable) module docs as Markdown, POD or
    manpage.

 *  Dropped useless features as --cat (one can always open the file)
    or display of partial documentation.

 *  Provided more rich usage hint.

 *  Rewrote tests with focus on more probable scenarios.

shellfu-get (whose only job is to print one of hardcoded strings depending
on $1) and shellfu.sh should be as light as possible.  Most of this code
is only useful (and intended) for library exploration, which is domain
of shellfu-doc.

A neat side-effect is, that in its new home, we don't have to be so
fussy about performance and footprinting, so we can afford luxury such
as putting code into functions and documenting them, using $1 for first
argument, using shellfu libraries(!)...

Needless to say, this is an incompatible CLI change (or it would have
half the effect), so we take the opportunity to re-do the CLI to match
the new home.