only load modules in STAMPY_MODULES

[ProducerMatt]

This PR adds a module whitelist in the environment variable $STAMPY_MODULES.

• move module-finding code to config.py
• add getenv_unique_list() function that checks for duplicates before returning
• only load modules in $STAMPY_MODULES, log the skipped modules
• add cases allowing $YOUTUBE_API_KEY to be empty (since this was needed to make Stampy function at all without it, if this should be separated let me know)

I decided to make a whitelist rather than a blacklist, since it means less surprises down the road for server admins updating their local Stampys. To get the original default-all behavior, set the variable to "ALL"

I've really only been testing with the paper search module, but I plan to go down the list of other modules and see what can be re-enabled. As I do so I may have more small changes to make in other modules.

[ProducerMatt]

Hang on, I think I screwed up the unset variable behavior. One moment please

[ProducerMatt]

Ok, it's working as intended now, my apologies.

[tayler6000]

MUST UPDATE .ENV ON SERVER BEFORE MERGE.