Make specifying contexts easier in commands

[zml2008]

For contexts that are provided in the form of a UUID, like GriefPrevention's claim contexts, it is difficult to add permissions in these contexts by hand. Some ideas:

1. Allow ContextCalculators to have a provideAlias method for contexts that will transform context aliases (and a completeAlias too I guess)
2. Add a --from-active=context[,context...] flag to all context-using commands that will get the current context value from what it is in the source's getActiveContexts.

[zml2008]

Another thing: Add context inheritance commands