Allow _clang-format as alternative to .clang-format config filename

Dotfiles are impractical on Windows. This makes clang-format search
for the style configuration file as '_clang-format' in addition to
the usual '.clang-format'. This is similar to how VIM searches for
'_vimrc' on Windows.

Differential Revision: http://llvm-reviews.chandlerc.com/D1629