emacs: Add 'guix-browse-license-url' command.

* emacs/guix-main.scm (%licenses): New variable.
(licenses, license-names, lookup-license, lookup-license-uri): New
procedures.
* emacs/guix-read.el (guix-license-names, guix-read-license-name): New
procedures.
* emacs/guix-license.el: New file.
(guix-lookup-license-url): New procedure.
(guix-browse-license-url): New command.
* emacs.am (ELFILES): Add new file.