CmdStanMCMC.summary: allow filtering of variables

[WardBrian]

This can use the -i argument from https://github.com/stan-dev/cmdstan/pull/1143

Some care will be needed if we want to support mapping from containers to all their indices, e.g. if the user can specify vars=['a'] for a vector[3] a; and that's equivalent to -i a.1 -i a.2 -i a.3