Config file support for clang-format, part 1.

Summary:
Added parseConfiguration method, which reads FormatStyle from YAML
string. This supports all FormatStyle fields and an additional BasedOnStyle
field, which can be used to specify base style.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D754