Allow recipes to modify the environment variables.

These need to be installed with platform brews vinstall option and are sourced as part of env.
Allow passing in extra args - at least to vinstall for options.
Don't re-visit repeated dependencies during vinstall.

Saves a lot of time for deep install trees.
Bugfix for env breakage with 1e9956f4a3c98ac3c29539e28720a7b191853182.
Merge pull request #17 from jmchilton/environments

Environments