Use configurable off-conditions instead of hardcoding them

The off-conditions '!&modifiable' and '&wrap' are kept as default.

The configuration variable g:excess_lines_off_when_textwidth_zero is now
deprecated.  You can add '&tw==0' to the new configuration variable
g:excess_lines_off_conditions.  For example:

    let g:excess_lines_off_conditions = [
            \ '!&modifiable', '&wrap', '&tw==0']

This makes this plugin more generally useful.  If it won't be
generalized further, you can copy it and rename the public stuff in case
you find another separate use-case.
Remove config g:excess_lines_off_when_textwidth_zero

If you want that behavior plus the previously hardcoded off-conditions,
add this to your configuration:

    let g:excess_lines_off_conditions = [
            \ '!&modifiable', '&wrap', '&tw==0']