Allow to add arguments to the diffing tool

[EmileSonneveld]

I wan't to ignore changes in indentation. An extra option would be nice for that, ex:

    "vcs_arguments": {
       "diff": "--ignore-space-at-eol",
       "git": "",
       "hg": "",
       "svn": "-x -w"
    }

Or is there already another way?

[primmer]

I use hg diff and would like to use a different revision -- not just what is uncommitted yet. I tried making an alias for hg diff so that you would call it with my args but the diff stops displaying with no indication of why.