Improves HTML diff table format for long line lengths.

[trevorshannon]

If very long lines are present in an HTML diff report, the HTML table layout can become less than ideal. One half of the diff can be very wide and the entire table can be wider than the screen.

The built-in difflib output includes a nowrap HTML attribute on the table cells containing diff text lines. However, wrapping diff lines is both useful and typical. See the below GitHub diff screenshot for an example:

This PR uses CSS to override that nowrap attribute and allow for sane line wrapping while also ensuring the left and right halves of the diff table are equal width.

Current rendering:

Table is too wide for the viewport, left side is much narrower than the right.

Proposed rendering:

Table fits within the viewport, both sides are equal width, long diff lines are wrapped without adversely affecting line numbering.

[Jamstah]

You should also update the CHANGELOG.md.

[trevorshannon]

oops, thanks I always forget that.