Add support for handling different configuration sources

[mfarr]

The app currently expects to find the configuration file in the app's root directory with a filename of evl-daemon.json. We need to be able to read configuration from several different sources.

Sources should include:

1. Command line switches
2. Environment variables
3. File on disk
   a. Path specified on command line
   b. Path specified in environment variable
   c. Default configuration
4. Default values

If multiple sources are used and sources have conflicting settings, the above priority should be used, ranked from highest priority (command line) to lowest (defaults).