Allow files to be excluded for AddMissingHeaderRule

[zsol]

Add an option to AddMissingHeaderRule so some files can be exempt from it.

Something like:

rule_config:
   AddMissingHeaderRule:
       path: pkg/*.py
       exclude:
           - pkg/uninteresting*.py
       header: |-
           # header line 1
           # header line 2

[zsol]

Although maybe #156 will help with this.

[jimmylai]

Make suppression comment lint-ignore: AddMissingHeaderRule works is a better and more consistent user experience. #156 is specific for ImportConstraintRule which doesn't seem related.