Implementing a tagedit-splice-tag function

Hi Magnar

I've been playing with tagedit, but one of my favourite paredit commands is `paredit-splice-sexp`, bound to `M-s`. I've implemented this for tagedit. I'm not sure how to write a test for this, and it doesn't reindent the child tags. Let me know what you think, or how I can improve this.

Thanks!