support installation via conda

[paxtonfitzpatrick]

requires implementing:

• parser object analogous to the pip parser in davos/parsers.py
• Davos method for conda installation (placeholder currently exists)
• initial check for whether conda executable exists with cached result
• updating some examples in the docs to show using conda as the installer

[paxtonfitzpatrick]

The README at be11816 contains some conda-related text, examples, links, etc. that should be added back in when support is implemented.

[paxtonfitzpatrick]

Code in conda.cli.install deals with actual installation of conda packages