Add optional appendExpandPrefixToSummaryText option

[tracyfloyd]

Sometimes I need to separate the expandPrefix from the link that actually triggers the expand (so I can treat the link as a block level element on it sown line, etc). This change adds an "appendExpandPrefixToSummaryText" option that if true will append the expandPrefix to the summary text instead of prepending it to the moreLabel.

[kswedberg]

Thanks for the pull request. I'm not sure, though, if it's worth adding this feature for such an edge case. Also, on line 415, it looks like you're still adding expandPrefix to summaryText, even though you're concatenating it onto ret in the previous line.