Change the shared problem matcher to a git subrepo

[peternewman]

The stuff that was added in #2 :

https://github.com/ingydotnet/git-subrepo/blob/master/Intro.pod

[peternewman]

Added via the following:
git subtree add --squash --prefix=codespell-problem-matcher https://github.com/codespell-project/codespell-problem-matcher.git master

Pulled:
git subtree pull --squash --prefix=codespell-problem-matcher https://github.com/codespell-project/codespell-problem-matcher.git master

If we ever need to push back:
git subtree push --prefix=--prefix=codespell-problem-matcher https://github.com/codespell-project/codespell-problem-matcher.git master