Add BEM block generating logic
Add help for block command
Merge branch 'feature/block' into develop
Change assumed conventions for BEM. See long description

Procss block generated a modifiers directory, but this
is not as convenient for ProCSS in the long run
since block and element modifiers would be mixed
in the modifiers directory.

Block modifiers are now assumed to be stored in partials
in a block branch, and elements are stored with their modifiers
in sub-branches of the 'elements' sub-branch generated with
'process block'.
Remove block command. Not needed
Remove block command docs
Add bem command with self-contained docs
Merge branch 'feature/bem' into develop