Change implicit textOnly fixes to only apply to indented block content

Fix for some of the spurious fixes mentioned in https://github.com/visionmedia/jade/pull/1036

I removed the initial test that ran before the replace operation, not sure if that did anything.

This is untested and I'm not *positive* it works (I'm a little fuzzy on how ^ and newlines interact in multiline mode, if regexes can still span multiple lines).