Allow easier modification of breadcrumb's header

D7 core's theme_breadcrumb() hard-codes the breadcrumb's header text and header classes. Currently, zen overrides that theme function in zen_breadcrumb(), but retains the hard-coded header info.

Some themes, like Austin, want to override just the header text and the header classes (so that the header is no longer invisible). But the only way they can do that now is to override the entire function and make tweaks. It would be nice if they could do that with a preprocess function instead.