Newline conversion when copy/pasting between editors

I was tracking down a failure in the tests on Windows a couple of days ago. During that period I noticed that all of them had to do with the way newlines were used in Vrapper. I fixed most test-cases by changing the way input files are read, but this got me worried that we would allow `\n` characters to be pasted in a file containing `\r\n`.

This patch set should replace any foreign newline characters with the ones specific to the editor. Vim seems to do this as well when you play around with `fileformat`s.

Also, because the conversion happens just before pasting, the system clipboard is also handled correctly in case newline characters are copied from an external file. It also doesn't matter what kind of newline we use in append registers as those get corrected.

I put it up for review because @keforbes has worked on this before and I might be missing something.