Hi,

In a EWW buffer, the user can use the 'eww-open-in-new-buffer' or "M-RET" to
open a URL in a new buffer and focus on the new buffer.

Sometimes people want to open the URLs in the background first and read them
later, so here is the 'eww-open-in-new-buffer-background' function,
binding to "C-<return>".

Example: using the eww to open the hack news and browse the topics,
open some topics in background with "C-<return>", and read them later.

Please help review the patch. Thanks.

Best Regards
Lin