Documentation - Better document available environment variable configuration

[sabrehagen]

If using Merriam Webster only, the default source will be Oxford, requiring a command line argument or config file to set the preferred source to Merriam Webster. Can support be added for configuring the preferred source through the environment please?

[sabrehagen]

Searching the source turns up DEFINE_APP_PREFERRED_SOURCE. Maybe this could be added to the documentation?

[Rican7]

Yea, as you mentioned, this is already possible via the DEFINE_APP_PREFERRED_SOURCE or DEFINE_APP_SOURCE environment variables (they have different behaviors).

However, you're right in that this could be better documented. I really should work on a man page actually...

[Rican7]

See #11