Help and usage messages

Specify that the help option causes the program to exit immediately.

Standardize order of options then positional arguments.

Formatting fixes; Help message fixes

Refactored code into multiple 'packages'. One day these may be
independently installable. For now it's just an organization method.

Rewrote main README and added per-package READMEs.

Also: bug fix on whiched and whichvi. They formerly propogated an error
if $EDITOR or $VISUAL were set to a non-existant executable. Also
improved the inline documentation.

Initial commit of cleaned-up dev branch