textdiff: implement support for colored output

Coloring is only possible for unified diffs

plots: exclude from module

The plots directory contains files that relatively large but unnecessary.
Excluding them reduces the size of the module a little.

textdiff/testdata: exclude from module

Excluding the testdata from the module reduces the size of the module
significantly (the testdata directory is 36 MiB in size).

diff, textdiff: add position information to Edits

Also rename fields in textdiff to use LineNo instead of Pos to disambiguate the
meaning.