Add --required-plugins and --allowed-plugins

[sigmavirus24]

Closes #283
Closes #488

[asottile]

I'm in the middle of reworking plugin loading and this goes in the other direction of what I'm doing

essentially the planned pipeline is:

1. initial opts
2. load all plugins
3. parse finalized opts
4. (then filtering/requiring plugins would land here)
5. then the rest of the execution pipeline

[sigmavirus24]

load all plugins

Loading them will break things and defeat the whole purpose of adding the options in the first place unless we're defining "load" differently.

[asottile]

load all plugins

Loading them will break things and defeat the whole purpose of adding the options in the first place unless we're defining "load" differently.

import and parse their options -- I realize there's still situations where that breaks as well but I think in those cases it's expected that the user would uninstall or isolate using a virtualenv. whereas I expect these options to be predominantly used to prevent the plugins from executing against files

[sihil]

Hello. This is an awesome feature, thank you all. Any idea when you'll cut a release that includes these options? Confusingly, they are on the latest documentation but not in 4.0.1 which seems to be the latest available unless I'm mistaken. Thank you!