Add a new 'empty' output mode

[matejak]

This output mode doesn't contain any Argbash-generated code. It is suitable for economical storage of Argbash-powered script if one believes to have reliable access to Argbash.

Feedback welcome - most importantly regarding naming, but also other aspects of this functionality.

Fixes: #186
FYI @nea89o

[kstrafe]

empty doesn't indicate to me that we excise code generated previously (if we're in a flow such as argbash X -o X). The code generator cog from python-cog lists this in its help:

-x Excise all the generated output without running the generators.

I think that is very clear. Should empty be renamed to something like excise?

[matejak]

Thanks, I am looking for exactly those suggestions.