[clang-tidy] Default options in modules.

Summary:
This patch allows modules to specify default options for the checks
defined in them. This way a sufficiently configurable check can be registered in
multiple modules with different default options. E.g. the SpacesBeforeComments
option may be set to 1 for the "llvm-namespace-comments" check and to 2 for the
"google-readability-namespace-comment" check without modifying or extending the
check code.

This patch also registers the google-readability-braces-around-statements check
with suitable defaults.

Reviewers: djasper

Reviewed By: djasper

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D5798