Add line deletion to StripTrailingWhitespace()

In addition to its existing functionality of removing trailing spaces
from the ends of lines, this change has the function remove lines at the
end of the file afterwards if they contain no non-whitespace characters,
based on its observations during the line iteration.

This uses the older VimL functions line() and col() in preference to the
newer winsaveview() and winrestview() to restore the cursor position
after the range :delete moves it, so that this will hopefully work even
on older versions of Vim; that is not yet tested.

I am surprised that there is no line deletion function to match e.g.
getline(), setline(), but it does seem to be the case:
<https://groups.google.com/d/msg/vim_use/TY9NmJXh8EU/iFjOUg68AekJ>

0 files changed, 0 insertions, 0 deletions