Embedded newlines

I needed to embed newlines in a block literal scalar, so I have a patch and tests to do that. The main thing was to get the regex split at the top of `_read_scalar` not to eat blank lines; then the various read loops have to deal with them.

I worry that I misunderstand how you're supposed to handle leading spaces in the middle of those block literals. I don't try to catch those and let the existing code detect errors there.

This is my first pull request ever, so please be gentle. :)