installing envstack files

[rsgalloway]

implement the equivalent of 'pip install' for envstack files, e.g.

$ pip install default.env

would copy the default.env file somewhere in $ENVPATH.

alternatively, perhaps just add an --install flag?

$ ./env/default.env --install

would do the same.

perhaps implement the equivalent of sysconfig.get_paths to get a list of ENVPATH locations to install .env files (see "purelib").

[rsgalloway]

consider just using a public alternative pypi index for projects that contain dist.json files