Expose new MOLECULE_COMMAND variable

[jhg03a]

When I'm developing my dependencies, some of the tasks inside are temporally expensive. If all I'm doing is linting, they're unnecessary. I'd like to see a new standard molecule environment variable added that exposes the command requested: check, converge, lint, prepare, etc. This enables me to add conditionals within my dependencies to tailor them more optimally to the needs of the command.