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.
To me, a justified paragraph is easier to read. Also takes up less space. If you think a remark needs to be separated from the rest (a la `# NOTE: Never call this in code that runs on Tuesdays.`), maybe leave a blank comment line in between?