Documentation for cxxopt

[LukeCz]

Added documentation and example for passing --cxxopt.

[erenon]

Hi, thanks for the contribution. It appears a different heading got pasted into the description body. Also, I do not understand the reference to --, it doesn't appear in the code example.

[LukeCz]

Hi, there reference to '--' is this:

bazel_clang_tidy/clang_tidy/clang_tidy.bzl

Line 46 in 4884c32

args.add("--")

.
It is used to separate clang-tidy flags from clang flags (https://clang.llvm.org/extra/clang-tidy/#using-clang-tidy). It took me a while reading Bazel (I'm not a Bazel expert) code to figure out how to pass compiler flag from bazelrc and this is why I added it to the guide.