Use cosmiconfig for config variety

[j0hnm4r5]

Fixes #330

[j0hnm4r5]

This seems to be working so far on my machine, albeit I'm not nesting any package.jsons or .xorcs. There's a few tests failing, but I think it's because they were written specifically to check for pkg-conf's handling of the config.

Annoyingly, while using a .xorc, you have to reload VSCode if you make any changes to the config; it seems like VSCode watches .eslintrc and package.json for changes, which is why a reload isn't necessary when using those. But that's not a big deal in the grand scheme of things (and those tat want to continue doing config in package.json won't be affected at all).

[aslafy-z]

Any chance you rebase your work?

[sindresorhus]

See: #279 (comment)

Can you also fix the merge conflict?

This will need some docs and test additions too.

[bunysae]

@j0hnm4r5
Do you want to do any on further work on this pull-request?