Add solcx entry point to allow commandline use

[iamdefinitelyahuman]

Something like solcx <version> <regular-args> to allow easy command line interaction with the multiple installed versions. If the given version isn't installed, return an error and suggest use of the --install flag.

[iamdefinitelyahuman]

This should be done using click. A good opportunity to learn it.

[step21]

could also use https://typer.tiangolo.com/