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 = "C:\Program Files (x86)\Meld\Meld.exe"

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"