Introduce a rough abstract base class for "linters which run programs and read the results"

Summary:
Ref T3186. We have about 50 linters which run programs and read the results, all of which have ad-hoc one-off custom config that isn't formalized anywhere.

Consolidate all this stuff into `ArcanistExternalLinter`, which is configurable through `.arclint` (although nothing supports this quite yet).

Extend CSSLint and Pep8Lint from `ArcanistExternalLinter`.

Add unit tests for both.

There are still some rough edges here, but it mostly seems to work pretty well.

Test Plan: Ran unit tests, hit some (most?) of the error cases.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran, Firehed

Maniphest Tasks: T3186

Differential Revision: https://secure.phabricator.com/D6800