Fix escaping of backslashes, make quote escaping play nice with allowesc...

...apes

New bug: When I made quote escaping less greedy yesterday, I forgot backslashes needed to keep escaping backslashes.

Old bug: When `allowescapes` or quote escaping escaped a backslash the other would use take that backslash as a mandate to escape the next character.

This is meant as a quick fix for the bug I introduced yesterday -- when `allowescapes` is selected, backslash-escaped double quotes will still raise an error.

@johnmyleswhite -- I've never had to use `allowescapes` or deal with c-style escapes at all in csvs -- anything else to change for that option? Should it be expected to play nice with quote escaping, etc.?