Add var and defun for removing trailing whitespace on save

[rearman]

Changes made in src/file-command.lisp

This change adds the variable delete-trailing-whitespace-on-writing-file and the defun clear-trailing-whitespace-on-write.
I tried to keep it in the same style as Add-Newline-at-EOF-on-Writing-File and add-newline-at-eof.

I tested by setting the variable to t, introducing trailing whitespace in multiple buffers, then saving using save-current-buffer and save-some-buffers to ensure the whitespace was removed in the proper places. I then did the same with the variable set to nil, and ensured that the whitespace was not removed.

[rearman]

Not sure what happened with the whitespace edits on that last commit, but the only change I intended to make was on line 73.

[cxxxr]

Thanks for the PR.
I was hoping to have this feature someday.