Paceholder when mask has optional characters

I was using the optional part mask (https://github.com/angular-ui/ui-utils/pull/21) and noted that the placeholder was not working as I expected (see this plunker: http://plnkr.co/edit/8d1rduAM7ag5CxOkFx1U?p=preview).

So I made this pull request with the code that solved this problem.