Improve paragraph filling of block literals

The functionality of this patch can be demonstrated on a small example. Consider the YAML document below:

    foo: >-
      block
      literal
    bar:

When point is inside the "block literal", pressing M-q (fill-paragraph) results in:

    foo: >- block literal bar:

This is probably not what most users want. With this patch, the result will look like in the following:

    foo: >-
      block literal
    bar: