added the excerpt_strip_p_tag config option to remove surrounding p tags...

... from extracted post excerpts

See https://github.com/mojombo/jekyll/issues/1160 for motivation. This allows us to do things like `<p>{{ post.excerpt }} <a href="{{ post.url }}">Read more</a></p>`, which is difficult/impossible to do currently with extracted excerpts, since the generated markdown will always contain a top-level `p` tag.

A dependence on nokogiri was added. If this is undesirable, I can try to throw together some string parsing stuff to just check for `p` tags manually (though having a full parse does seems preferable, especially for cases like `"<p>One paragraph.</p><p>Two paragraphs.</p>"`), or see if there's a way to achieve this with existing dependencies (I'm a bit out of practice with ruby)