whiched and whichvi update

whiched and whichvi should exit as an error if no programs are named as
arguments.

This change was actually accidentally included in an earlier commit for
whiched.

whiched and whichvi now respect the quiet option

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.

Specify full path to POSIX utilities; Clean up scripts' output

Initial commit of cleaned-up dev branch