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 sure "host" is an improvement. It's true that "host" is usually what you call the thing that plugins plug into, but I think that's confusing given the way we actually use them. Should we use a different word instead of "plugin"?