Improve LINT_DOS_NEWLINE behavior

Summary: Improve the behavior of ArcanistTextLinter in dealing with LINT_DOS_NEWLINE. In particular, provide replacement text so that these linter issues can be autofixed.

Test Plan: Added a unit test.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10472