Misc. changes from 1.0 or prior:
Common line prefix is now computed without considering blank
lines, so blank lines need not have any indentation on them just
to "make things work."
The tricky case where all lines have a common prefix, but it's not
entirely composed of whitespace, now properly handled. This is
useful for lines that are already "quoted" such as with leading
``"|"`` or ``">"`` symbols (common in Markdown and old-school
email usage styles).
``textlines()`` is now somewhat superfluous, now that ``lines()``
has a ``join`` kwarg. But you may prefer it for the implicit
indication that it's turning lines into text.