Pass arguments to devenv up through to implementation

[jmackie]

I'm using the process-compose implementation and I'd like to be able to run specific processes sometimes.

For example, suppose I have a server process and a postgres process, there are times when I just want to start the postgres process for tests. Something like:

devenv up postgres

Which is hopefully what this PR adds support for.

I must admit I'm not that familiar with the other implementations (honcho et al.) but I'm assuming the same change applies for them?

[domenkozar]

Seems like a whitespace slipped through