better argument parsing

* don't parse arguments unless the subcommand is one dfm handles
* better organization of subcommand code

fixes #7
added import help
new subcommand 'help'

* reorganized pod content and use Pod::Usage to make it easier to get
  help for individual commands
* 'help' all by itself has a more concise output, like git's help
* clean up a few unnecessary exits (seth--)
* help is now the default subcommand (seth--)

closes #5
support $DFM_REPO outside of $HOME
make bash loader configure work with repos outside of $HOME