Eval list- and dict-typed option values, instead of JSON-parsing them.

This is to support list concatenation syntax in config files, e.g.,

dev_opts: %(basic_opts)s + %(big_mem_opts)s + ['-ldebug']

This is a legitimate use case that we actually have, but that I overlooked
when I made the JSON requirement in 8c3b5b33e8cabdf599ed1332067283ffc067a13a.

Testing Done:
Ran options unit tests.

CI baking.

Reviewed at https://rbcommons.com/s/twitter/r/1443/
Sync of https://github.com/pantsbuild/pants/commit/676c0e80b90e2e6293f7204db8cb57a412c53d8f