Sanitize inputs to bookmark-cmd (#591)

Newsbeuter didn't properly shell-escape the arguments passed to
bookmarking command, which allows a remote attacker to perform remote
code execution by crafting an RSS item whose title and/or URL contain
something interpretable by the shell (most notably subshell
invocations.)

This has been reported by Jeriko One <jeriko.one@gmx.us>, complete with
PoC and a patch.

This vulnerability was assigned CVE-2017-12904.