allow csv fields to have multiple lines

This is a potential fix for #143

I sometimes keep small amounts of content is csv files, and the content has newlines in it. As I mention in #143, the newlines get stripped out, even though they are quoted.
I noticed that the problem goes away when you give the csv reader an actual stream instead of a list of strings, so that is my offered solution. I would think it might be better to just change the API so `import_set` takes a stream instead of a string, but I assume there are good reasons to have it this way.