Open current xkcd in external browser

Two new functions: open the strip at the xkcd website in default
browser, and open the same strip at the xkcd explanation
website. Defined key bindings 'o' and 'e' for these functions.

As we just noticed with animated gif at #1331, there will always be surprises. Linking to the website can help to understand what is going on. For good measure, I added a link to the Explain XKCD website.

It is not anymore pure, self contained  emacs, but I think it is justified.