Introduce .gitattributes file

PR https://github.com/Microsoft/TypeScript/pull/962 was reverted in commit https://github.com/Microsoft/TypeScript/commit/e179e0565cfb90c30dc219e49599547948bb2e10. This PR aims to fix the reported issues.

Refer to GitHub's [dealing with line endings](https://help.github.com/articles/dealing-with-line-endings/) article for more information how to properly treat line endings in a project.

Ping @mhegazy