Figure out the common prefix used in a paragraph automatically

Instead of only working with the line comment prefix, we see what prefix
is on all of the lines, and preserve that.  This mostly reverts the
previous commit.

Ref. #1.
Paragraphs are delimited by lines without alphanumeric characters

Such lines include block comment delimiters and some Markdown notation.

Ref. #1.
Pay attention to line comments when selecting paragraph

Closes #1.