Allow specifying coverage flags via a yaml file

[liamappelbe]

Spin off of #510

It's a common pattern for Dart tools to read the default values for all their command line flags from yaml. This lets developers specify flag values once per project, and not have to remember/retype them each time. We should do this for package:coverage's flags.

There are a couple of open questions:

1. Should we read the config from a field in pubspec.yaml, or define a new coverage.yaml. Ffigen supports both, the analyzer and dart test only support the latter. I'd prefer to have our own yaml config.
2. We have multiple tools (collect_coverage, format_coverage, run_and_collect, test_with_coverage, and more to come: Add a tool to find uncovered files #529). Do any of their flags have the same name? If not we can just mash them all together into one big config. If there are collisions we might need to group the flags by tool. We'll need to audit all the tool's command line flags and figure out which ones should be exposed in this config, and if any collide.