--inject flag

[abrahammurciano]

I thought it would also be a good idea to have an --inject flag for install, e.g.

$ condax install foo --inject python=3.8 --inject bar

which would install foo as a primary package with py3.4 and bar in that same environment.

This would be a shorthand for

$ condax install foo
$ condax inject --name foo python=3.8
$ condax inject --name foo bar