Help and usage messages

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

Standardize order of options then positional arguments.

Helptext updates

The more honest description of these programs is [OPTIONS] before
PROGRAM. Options like quiet and verbose do behave differently if they
follow.

whichhead fix

The helptext incorrectly claimed that --number=N was supported.

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