Add line break checks

[iafan]

I'd like us to have the following line break checks implemented in order to deal better with multiline (plaintext) units that can be split into multiple paragraphs:

1. single line breaks — this check needs to count only single line breaks both in source and target, i.e. LF, and trigger if counts don't match.
2. double line breaks — this check needs to count only double line breaks both in source and target, i.e. LF LF, and trigger if counts don't match.
3. 3+ line breaks — this check needs to count instances of only 3 or more subsequent line breaks both in source and target, and trigger if counts don't match.

The reason for distinguishing between these cases is to give translators a better idea on where to look at, and to understand that multiple line breaks have special meaning (i.e. they are used to break text into paragraphs).