Added keys to mpd_now for querying the length of a song and the current time

[jepugs]

I added functionality to the mpd widget by extending the mpd_now table with keys mpd_now.elapsed and mpd_now.time, which give the current time in the song and the total length of the current song respectively, in seconds. This is some code from my rc.lua that uses the widget:

-- Represent a number of seconds as a string of minutes:seconds.
function format_time(s)
   return string.format("%d:%.2d", math.floor(s/60), s%60)
end

-- Add an mpd widget
mpdwidget = lain.widgets.mpd({
      settings = function()
         local artist = mpd_now.artist
         local title = mpd_now.title
         local state = ""
         local time = ""
         if mpd_now.time ~= "N/A" and mpd_now.elapsed ~= "N/A" then
            time = string.format("(%s/%s)", format_time(mpd_now.elapsed),
            format_time(mpd_now.time))
         end
         if mpd_now.state == "pause" then
            state = "[Paused]"
         elseif mpd_now.state == "play" then
            state = "[Playing]"
         end
         widget:set_markup(string.format(" %s %s - %s %s ", state, artist, title, time))
      end
})

Example output: [Playing] Danger Mouse - What More Can I Say (1:29/4:25)