Redirect xdg-open output to dev/null to avoid START... output

When using tmux-open's o command on Linux, xdg-open will output the following to the tmux pane in which it was executed:

    START /usr/lib/firefox/firefox "http://example.com"

Redirecting to /dev/null avoids this.

Ideally, xdg-open would have an option to suppress this output, but it does not.  This redirect applies to osx's open command as well, but I don't think that should be an issue.