Allow global excludes to be specified in `.arclint`.

Summary: Currently, paths to be excluded from linting need to be specified for each linter individually. This is a pain for projects that are using even a moderate number of linters and which have common paths which should be excluded from linting completely.

Test Plan: Unfortunately, it's hard to test this sort of stuff. I cloned the `arclint-examples` repository and tested my changes there,

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9054