fix: less eager help command execution

[bcoe]

Now that yargs is being used to build chat bots in a few places, the eager help command feels like more of a hinderance than a help; It's relatively rare that help ends a sentence, but it's fairly frequent for help to show up somewhere in a natural language string (triggering help output in a chat bot).

This is a breaking change:

• we no longer allow a single character help command to trigger help output.
• a help command must now be at the end of a sequence of commands to trigger output.

fixes #676

BREAKING CHANGE: help command now only works if it's the las command in a list.

CC: @zkat