[clang-tidy] Make format style customizable

Summary: I came across an outstanding FIXME to make the format style customizable. Inspired by the include fixer, I added an new option `-style` to configure the fallback style in case no clang-format configuration file is found. The default remains "llvm". 

Reviewers: Prazek, aaron.ballman, hokein, alexfh

Subscribers: cfe-commits, malcolm.parsons

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D27142

llvm-svn: 288258