Add env filter property to commands

[ShinyHobo]

We should probably add a property to each command that denotes which environments are supported so we can just grab a subset automatically.

Originally posted by @ShinyHobo in #42 (comment)

[un0btanium]

Hm, I initially like the idea, but thinking about it some more, it might be best not to. One of benefits of the strategy pattern is that it doesnt directly affect any of the other code. So one can easily add new strategies or remove existing strategies without having to touch any of the code elsewhere (like the commands if we were to introduce those properties).

On another note: Thinking about it some more, maybe the ideal approach would be to even explicitly state each command in each strategy, rather than reading in all commands from the folder first and then removing those shouldnt be available. Because, when doing it this way, when adding new commands, one would have to manually add this command only to the strategies that require it. Otherwise (the way it is currently implemented) a new command will automatically be added to all strategies and then the developer would have to check each strategy to remove it if it doesnt require it. This seems to be less error-prone to me. But please let me know what you think.

[ShinyHobo]

Being explicit is probably the best strategy, yes. We will want to check the list against the available commands, though, as a sanity check.