Skip runs creation via special commit messages

[sgotti]

Add option to handle special commit messages like [ci skip] to skip runs creation

[cirelli94]

I think this should be closed thanks to #90 !

[sgotti]

Currently implemented in #90. There's just the "issue" that it won't work when merging a PR since the commit is something like "merging pull request...". This could be considered the right behavior (and should be documented) or we should find a way to handle also such case.