Allow captures to be split across lines.

If you have a lot of them, it's nice to be able to:

  caps: 1=markup.link.text.markdown 2=markup.link.text.markdown \
        4=markup.link.title.markdown 5=markup.link.text.markdown

I'm considering also allowing a nested dictionary format:

  caps:
   1: markup.link.text.markdown
   2: markup.link.text.markdown
   4: markup.link.title.markdown
   5: markup.link.text.markdown

This is particularly useful for the massive regexps one finds in tmLanguage
files in the wild, which often have ten or more capture groups.