Add support for Meld diff/merge tool

Add built-in support for Meld diff/merge tool to SourceTree in the Diff options/preferences dialog.

I currently have to set `External Diff Tool` to `Custom` and the following.

Command = /Applications/Meld.app/Contents/MacOS/Meld

Arguments = "$LOCAL" "$REMOTE"

The above could possibly do with some back-slahses for robustness?

For `External Merge Tool`:

Arguments = "$LOCAL" "$BASE" "$REMOTE" --auto-merge --output="$MERGED"