Markdown-ify the bladeRF-cli interactive help

Note: This pull request replaces #270, which was ending up somewhere beyond 6 commits.  This should clean things up a bit.  -rt

Adds a new include, doc/cmd_help.h.  This is generated by
pandoc from doc/interactive-help.md.  It's ugly, but that's
why it's programatically generated.

CMake will either generate a new cmd_help.{h,man} or copy them
in from cmd_help.{h,man}.in if pandoc isn't available.

Update cmd.c to use all of these new #defines.  (It looks
cleaner now...!)

Remove the old get-interactive-commands.bash, which is no longer
a part of the build process.  pandoc also generates a man page
snippet for insertion by help2man.

Also some editorial/spelling clarifications/corrections, and
adjustments for formatting.