Allow specifying config file with cli option (#170)

[adamblake]

This adds a config option --config which accepts a file path to an alternative configuration file (e.g. chartpress.dev.yaml). The PR passes all existing tests and adds a few new assertions to test the new code.

The use-case I am supporting is alternative chartpress files for different deployment contexts. For example, I have a local development context for a JupyterHub that has a couple extra convenience services, and those need different build arguments for the Hub Docker image. The way that I manage this currently is to temporarily rewrite chartpress.yaml with the updated values before calling it, but I would instead like to just load a different configuration file.

(The ideal implementation would be something more akin to how you can layer different configs on like you can with Helm values, but that's something I don't really have time to develop.)

After checking the issue lists, I believe this would also close #170

[adamblake]

@manics this is now refactored per your requests -- thanks!