Add preserve empty lines option

[mkozono]

I took a stab at Could you make an option to let one empty row when there are multiple empty rows? #5.

This is branched off of #41 because it helped fix this test in this PR, so that's why the first three commits seem unrelated. I can rebase this branch onto master if #41 gets merged.

Side notes

• I also got this to work with a separate mapping above new_line, as suggested, but it turned out to be slightly simpler this way anyway.
• The test in the last commit never fails in this PR but I thought I'd add it since it catches a bug I created in my other implementation, so it may make a future refactor a little safer.

[threedaymonk]

This is fantastic. I'm going to be busy for the next few days, but I'll try to take a proper look at this (and #41) next week.

Thanks!

[threedaymonk]

I've rebased this and squashed the test and implementation commits into two (one for test, one for implementation) and merge that into master. Thanks!