Add support for 'preset' option in MANGOHUD_CONFIG

[ForTheReallys]

Sorry for the long wait. Took some time to focus on some life stuff.

This PR adds support for passing 'preset' using the environment variable. It additionally has a few (hopefully desirable) side effects:

• Consistently handle options when passed through MANGOHUD_CONFIG and through the config file (e.g. "full")
• Better handle read_cfg (no longer required as the first argument in MANGOHUD_CONFIG)
• Because of the previous inconsistency when using "full" through the env and the config file, I chose to use the same options that were previously used in the config file instead of the environment variable.

Let me know your thoughts

Fixes #1129

[flightlessmango]

All of it seems like improvements to me. Thanks for fixing this!