Add a `--with` option to `rvx`

[indirect]

Running a command with rvx sometimes needs additional gems that aren't dependencies. Real world example from yesterday: I wanted to use the sequel gem to copy a sqlite database to a postgres database. That means I need to be able to run sequel from the gem named sequel but with the pg gem at the same time.

Running rvx --with pg sequel or rv tool run --with pg sequel should:

1. Install the sequel gem and all its dependencies
2. Install the pg gem and all of its dependencies into the same set of gems
3. Run the sequel command from the sequel gem, with the pg gem available