Add configuration using files and environment variables.

Doorstop should accept settings in three different ways:

- command-line options (current)
- configuration file
- environment variables

[configman](https://github.com/mozilla/configman) purports to do [exactly this](http://pyvideo.org/video/2846/configman-the-grand-unified-theorem-of-configur) and can [apparently](http://www.twobraids.com/2014/05/crouching-argparse-hidden-configman.html) work with `argparse`.

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TODO:

- [ ] research using `configman`
- [ ] determine the order of precedence for configuration options
- [ ] determine which configuration filenames will be accepted
- [ ] implement feature 
    - [ ] support server settings from #117