volume: get the normalized volume like alsamixer and DE's indicator

Or the percentage value will be different from alsamixer or desktop
environment's indicator, which confuses users.

Quote from amixer's manpage, "Use the mapped volume for evaluating the
percentage representation like alsamixer, to be more natural for human
ear."

ref:
http://git.alsa-project.org/?p=alsa-utils.git;a=blob;f=alsamixer/volume_mapping.c

Signed-off-by: Adam Lee <adam8157@gmail.com>
Signed-off-by: Adrian C. (anrxc) <anrxc@sysphere.org>

widgets/volume.lua

@@ -26,7 +26,7 @@ local function worker(format, warg)
     }
 
     -- Get mixer control contents
-    local f = io.popen("amixer get " .. warg)
+    local f = io.popen("amixer -M get " .. warg)
     local mixer = f:read("*all")
     f:close()