Ignore files from coverage by path pattern

[shouichi]

Currently, // coverage:ignore-file is available for ignoring the whole file from coverage but some tools (such as built_value_generator) do not generate files with that meta-comment. It would be nice if we can ignore generated files from coverage by specifying something like --exclude *.g.dart. We could fix tools to add the meta-comment but it may make more sense to let developers choose whether to exclude generated files from coverage or not. Thank you.