Remove limitation about order of inputs

[MarcelRobitaille]

From the readme:

in the main task command the input variables should appear (left to right) in order of dependence

In theory, it should be possible to remove this limitation. In the variable resolver, when encountering an input that does not exist in the UserInputContext, it should be possible to execute that input in that moment. The arguments to CommandHandler are args (can be looked up through other means), userInputContext (we have it in the variable resolver), context (we get it from activate, so it could be piped through to the variable resolver), and subprocess (just an import).