Feature/mark custom types

[JensHeinrich]

EDIT by @Kludex:

• Closes Mark methods on arbitrary classes  #106

[JensHeinrich]

• Rebase and reorder to make the "Fixes" commit the last commit after fixing the feature

[slafs]

Nice! Thank you. This will be useful for us too! 🙇 (haven't reviewed this PR though)

Just noticed that the rule code should probably be BP009 (or BP011 maybe). The last current working one is BP008, but BP009 and BP010 are only mentioned in the README, they are not-implemented and commented out in the README. Looks like the last time a new mod was added (BP008), Marcelo incremented the number of commented out mods and used first "free" one. See #90.

[JensHeinrich]

Nice! Thank you. This will be useful for us too! 🙇 (haven't reviewed this PR though)

Just noticed that the rule code should probably be BP009 (or BP011 maybe). The last current working one is BP008, but BP009 and BP010 are only mentioned in the README, they are not-implemented and commented out in the README. Looks like the last time a new mod was added (BP008), Marcelo incremented the number of commented out mods and used first "free" one. See #90.

Ok, thought I would just append to the commented one^^

[JensHeinrich]

Nice! Thank you. This will be useful for us too! 🙇 (haven't reviewed this PR though)

Just noticed that the rule code should probably be BP009 (or BP011 maybe). The last current working one is BP008, but BP009 and BP010 are only mentioned in the README, they are not-implemented and commented out in the README. Looks like the last time a new mod was added (BP008), Marcelo incremented the number of commented out mods and used first "free" one. See #90.

Updated the numbering

[Kludex]

We need to add a test for when there's already a TODO comment.