Added `ArcanistTextLinter::LINT_BOF_WHITESPACE` and `ArcanistTextLinter::LINT_EOF_WHITESPACE`

Summary: This might not be universally desireable, but I found myself writing an additional linter (which I had called `WhitespaceTextLinter`) for the sake of these two linter tests. I figured it may be of use upstream, and so I decided to submit it as a diff. I won't be offended if it is rejected however.

Test Plan: `arc lint` and `arc unit` are both okay with it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7957