Rule could-be-keyword-tags: Ignore some builtin tags

[Lakitna]

could-be-keyword-tags highlights situations where all keywords in a file use the same tag. These tags can be consolidated in the Settings section instead.

In my code base, it keeps raising situations where all keywords use robot:flatten or robot:private. These tags are instructions for the Robot Framework runtime. Because of this, I don't think they should be consolidated like other tags. I want to keep them explicit.

Potential fixes:

1. Keep it the way it is.
2. Always ignore robot:* tags (full list)
3. Add a configurable parameter to this rule that allows users to exclude specific tags.

[bhirsz]

I think 2. is the best.

3. could be added regardless, but very low priority