Follow-up: Use cosmiconfig

[aslafy-z]

Following up #279 and #331

This PR reuse #331 commits (thanks @j0hnm4r5 !). I opened it to address once and for all a very anoying issue when using lint-staged to trigger xo. I made all the tests pass, let me know if I need to add some.

Summary:

• use cosmiconfig instead of pkg-conf to fetch xo config
• give a relative path to findApplicableOverrides

Closes #279
Closes #331

Fixes #330

[sindresorhus]

See: #279 (comment)

[pvdlg]

See #279 (comment). That doesn't seems to solve that problem as you still load the configuration relative to cwd here and not relative to the path of the file being linted.