[Feature request]: please add a search path for patterns

[tmbdev]

What do you need?

Copying files into the ~/.config/fabric/patterns directory is kind of messy and doesn't work well with upgrades or dotfile maintenance scripts.

Why not add a search path for patterns as an environment variable, say FABRIC_PATTERN_DIRECTORY? By default, it could be ~/.config/fabric/custom_patterns:~/.config/fabric/patterns, but users could set it to something else.

[Starfish-Fujieda]

I'd like to be able to never see the ones I'll never use.