RFE: Be able to specify multiple packages to install

[opoplawski]

conda's depsolving is pretty strange, and if you ask to install packages one at a time you can get pretty different results and a lot of churn among dependencies compared to when you request all of the packages you want at once.

First decision is how to specify the list of packages and perhaps versions. My preference would be to try to match a module like dnf as much as possible. This would be a big change though - as I think we would want to get rid of the separate version argument and have users pass in the version specification in name (as is done in dnf). Then name could be a comma separated string or a list of packages.