Provide environment variables

[pitrou]

It would be useful for asv to set up environment variables when running either the install steps (setup.py build or pip install) or the benchmarks themselves.

I'm thinking something like:

• ASV=true
• ASV_PROJECT=<project name>
• ASV_COMMIT=<git commit id>
• ASV_ENV_NAME=conda-py3.6-etc
• ASV_ENV_PATH=<path to .asv/env/envhash>
• ASV_ENV_TYPE=conda

[pitrou]

This may also allow implementing support for #78 more easily.