Escapes newline character in cell data, fixes multi-line text exports

[mikeharty]

I noticed that multi-line text data was being output with actual linebreaks, rather than \n, which isn't valid JSON. This fixes that.

[abhibeckert]

Thanks, looks great. I'll test/merge this into master in a few days.

[mikeharty]

Cool - I eventually discovered that the tab character needed to be escaped as well, added a second commit.