Add ability to read from a global config file

[drose-shopify]

Allows the use of a global config file defined at ~/.solargraph.yml

#197

[drose-shopify]

I noticed the linked issue asks for the global config to be in ~/.config/solargraph/config.yml. Is there a preference to where the global config should live?

[castwide]

Thanks, @drose-shopify!

Using ~/.solargraph.yml is risky because it could make the language server treat the entire home directory as a single workspace, which is almost certainly not the user's intent. I'd prefer either ~/.config/solargraph/config.yml (common convention) or ~/.solargraph/config.yml (reuse of existing data directory). Maybe it could also be configurable with an environment variable, like #125 did with the document cache.

[drose-shopify]

Ahhh fair enough, I didn't think about that. Also <3 the idea of using an environment variable to set the path.

[castwide]

Awesome, thanks again! Merging to include in version 0.35.