Add support for --features=external_include_paths

[filippobrizzi]

I would like to add an option so that if bazel build is run with option --features=external_include_paths this line:

args.add_all(compilation_context.external_includes.to_list(), before_each = "-isystem")

becomes:

args.add_all(compilation_context.external_includes.to_list(), before_each = "-I")

I will be happy to contribute, but I would need some indication, as I am new to Bazel and I am not sure how I could read the features value and what it the best options to pass it to the aspect.

Thank you