adding "details" to tag whitelist

Apologies if this pull request is to the wrong branch, or in any fashion inappropriate.  It's to support this feature request: https://github.com/jch/html-pipeline/issues/138  which comes from my desire to use summary/detail tag combinations in github markdown.  I'm assuming that if the summary tag is acceptable, then the detail tag is okay too?

I've run the tests locally and they all pass (for ruby-2.1.2).  Hoping that this can get into a new version and then into github markup so that everyone can enjoy details/summary goodness in their Github READMEs :-)

I'm just installing other supported rubies to check tests there as well ... will update this pull request if anything crops up