Feature to change name of invocant variable on import

I just hacked this up because I was using M::S in some Mojolicious code and find that I really prefer using variable names that reflect what the invocant is, like $app and $c (for controller), etc.

It's just the barest amount of code to make it work right now - Let me know what you think - if it would be useful to add to M::S, I will go ahead and add validation for the variable name (right now the user could specify *anything*, which may or may not be a good thing...)

I'm also wondering if it's better to have the user specify the name of the variable *without* the sigil, since I don't see how using anything other than a scalar would be useful in this sort of case. I don't know if there's any such restriction on when the invocant is specified in a method sig.