D3650: serve: add an option to open in the default browser

[Yuya Nishihara]

(Resend because I forgot to remove the "someone wrote:" line again, sorry.)

>   For laziness,  this patch could be also useful. It is also better for text shell beginners, because they may not know that they can combine commands like that (even if it is basic).

For laziness, we can add it to `[alias]`. I'm not a fan of adding lots of
one-off options since that's the source of man page bloat.

How about document the xdg-open example in `hg help serve -v`?