separate plugins into discrete commands

Separate plugin types by directory and commands by individual scripts. Support for a command is determined by the presence of a script with an appropriate name. Unfortunately, only a prefix can be used, since extensions are necessary on some platforms.
I'm not a fan of justified text, especially per vim's default style. Since I obviously want to keep the line length down, I simply begin each new thought (i.e., each sentence) on a new line, and break lines beyond column 80 as usual.