Improve usability of external command support.

As a foreword, I suggest reading this article.

Said that, I would suggest (as a new iteration of external command support) to implement two new placeholders.

{file} - full path to the saved image. Equivalent to the existing {0} but fully escaped already, so not need to be quoted.
{title} - an optional image title as a commandline parameter. Also fully escaped.
If this placeholder is given, Greenshot should show a form (may be with image preview) asking to fill the image title before submitting the capture to the program.

Another addition I would suggest is an external command configuration option "image description", which would ask user to fill a longer accompanying text (see "title" placeholder), which will then be passed to the external program as STDIN stream.