Change `build_tag_filters` and `test_tag_filters` options to accumulate uses

Description of the feature request:

Change --*_tag_filters options to accumulate multiple uses, or provide some alternative mechanism for achieving the same thing.

Many of bazel's command line rules accumulate multiple uses. This is very useful for providing default configuration in a repository .bazelrc which users can then build upon or modify in their own bazelrc files and with command-line syntax. Currently, these tag filter rules don't allow for the same sort of use.

Feature requests: what underlying problem are you trying to solve with this feature?

For example, we have a Makefile rule that runs flake8 linting test before all other tests (because flake8 fails quickly if it fails), which looks something like this:

test:
	bazel test //... --test_tag_filters=+flake8
	bazel test //... --test_tag_filters=-flake8

This works great until you start adding other --test_tag_filters to your .bazelrc (either a default config one or your user one). These invocations totally ignore those options. It would be very nice if they simply accumulated, like --action_env, for example.

Perhaps the extra syntax --build_tag_filters=^tag1,+tag2,-tag3 could be used where ^tag1 means "ignore any previous mentions of tag in this list". This would enable .bazelrc filter lists to include -tag1 (or +tag1), while letting the command line invocation override that.

What's the output of bazel info release?

release 0.22.0