Support multiple config files via CLI

[JSKenyon]

It would be useful to be able to specify multiple config files via the CLI. The logic would be that, given cli file1 file2 file3, their config would be merged such that the last specified file has the highest priority. This should be relatively simple as merging config is a very standard operation in OmegaConf.

The motivation for this is that it would make partial config changes less cumbersome i.e. in my use case, I want to tamper with the backend options, but don't want to create dozens of almost identical config files. With this change, the small handful of backend options I want to tamper with could be placed in a separate file and then simply added via the cli.

[o-smirnov]

An excellent idea. Works on issue-191 now, will issue a PR shortly.