Ignore emacs flymake-mode temp files.

[Flymake][mode-page] is built-in widely used emacs mode that performs on-the-fly syntax check. It [creates temporary copies][path-convention] of checked files by adding ```_flymake``` before the file extension, so we can safely ignore these copies.

[mode-page]: http://www.emacswiki.org/emacs/FlyMake
[path-convention]: http://www.emacswiki.org/emacs/FlyMake#toc14