Preserve comments on merge.

[j-bennet]

This fixes #171. If we're merging config2 into config1, then values from config2, if present, should override values from config1. The same thing needs to happen with comments, but currently it does not.

Related to dbcli/pgcli#1240.

• Run the tests and make sure they're passing (invoke test).
• Check for violations of code conventions (invoke check).
• Make sure the documentation builds without errors (invoke build --docs).

I have an error running invoke check and could not resolve it by googling, would appreciate advice on that:

https://gist.github.com/j-bennet/392e90ed75873a69b465e32dd6763476

[j-bennet]

@EliAndrewC or @robdennis, any chance you'd have the time to review?