Improve folding

Hi,

the following changes should improve the handling of folding markers. I tried them on a slightly extended version of the [example file](https://en.wikipedia.org/wiki/YAML#Sample_document) from Wikipedia which worked fine. However, I am pretty sure that there exist cases I did not consider. So if anyone has example code that does not work, please just post it below. I might just try to fix the folding for this code too.

Kind regards,
  René

P.S.: The changes should hopefully (more or less) fix issue #2 (“YAML code folding isn't working in TextMate 2”) and make pull request #3 “Update Indenting.plist” unnecessary.