FEATURE_ environment variables

[ilanschnell]

In this PR, I want to make it easier to create recipes with support features. By setting FEATURE_* environment variables, the user can select whatever feature they need for building a recipe with and without that feature enabled.
For example, having FEATURE_DEBUG=0 will add the debug variable (set to False) to the selector namespace, and set DEBUG=0 in the environment of the build script.

It is also important to see that during building and testing, the feature is enabled (when set to 1).

[msarahan]

This is a good idea, much in line with #848. I don't think environment variables are the best way to do this, though. I'm going to ponder this, review the code, and try to incorporate it with my efforts to incorporate #848 ( #966 )

[msarahan]

Thanks @ilanschnell - merging.

For those observing, we had a meeting where I asked Ilan not to generalize this PR, so that it did not conflict in spirit with the discussions from #848 et al. This PR is necessary to push forward with our internal goal of making our legacy build recipes into conda recipes. We anticipate that the syntax that this PR supports will eventually be supplanted by the final implementation of proposals from earlier discussion about customizing build pinning, features/variants, compilers, and the rest.

[ilanschnell]

Thanks for merging!