Customize sign characters

[ricobl]

It would be nice to have an option to customize the signs.

I've found the signs for deleted lines on https://github.com/akiomik/git-gutter-vim more appropriate, but vim-gitgutter has nicer features. :)

The current delete sign makes me think that the modification is in a different line, git-gutter-vim solves this by marking one line before and after the deletion.

I think the sign definition is here:
https://github.com/akiomik/git-gutter-vim/blob/master/autoload/gitgutter.vim#L36

[airblade]

I'll add the ability to customise the display of the current signs as per #28.

However I'm not going to add two signs for deletion. I can see the appeal but I prefer my current approach.