Added snippets for panel-group, panel, panel-heading, panel-title, and p...

Thanks a lot for your work on your bootstrap-snippets fork. I added snippets at the bottom of html.snippets for the bootstrap "panel" component. There are snippets for each of the classes panel-group, panel, panel-heading, panel-title, and panel-body. (As per your approach the trigger string is the class name with the hyphen removed.) The one weird thing is the snippet for panel-body. "panel-body" is actually not the class of the outermost <div> in the snippet, but rather its first child. It seemed, though, the most descriptive and semantic way of referring to what the snippet does.