[clang-tidy] Test username substitution in TODO() check.

Summary:
This patch depends on D5501.
Check user name substitution more specifically by specifying it via the new
-config= option.
This also lets clang-tidy tests specify clang-tidy options before the '--'
when using check_clang_tidy_fix.sh.

Reviewers: klimek

Reviewed By: klimek

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D5502