Add help pages for individual cell and ssj commands

[fatcerberus]

The command-line help for Cell and SSj has always been intentionally high-level and barebones, intended to get users started quickly and give a bird’s-eye view of what the tools are capable of. If you need more details, the man page is there—but there’s something to be said for being able to get help on-demand, without leaving the command line.

The proposal here is to be able to type on the command line something like cell help build or ssj help host to get more details about a given mode, as well as which specific options are valid for that mode.