volume: suggest appending card ID or other options

Widget type takes the mixer control as an argument, i.e. "PCM" but can
take optional amixer options, i.e. "PCM -c 1".

README

@@ -264,7 +264,8 @@ vicious.widgets.mpd
 
 vicious.widgets.volume
   - provides volume levels of requested ALSA mixers
-  - takes the ALSA channel as an argument, i.e. "Master"
+  - takes the ALSA mixer control as an argument, i.e. "Master",
+    optionally append the card ID or other options, i.e. "PCM -c 0"
   - returns 1st value as the volume level of the requested channel
 
 vicious.widgets.weather

volume.lua

@@ -19,9 +19,9 @@ module("vicious.volume")
 
 
 -- {{{ Volume widget type
-local function worker(format, channel)
-    -- Get mixer data
-    local f = io.popen("amixer get " .. channel)
+local function worker(format, warg)
+    -- Get mixer control contents
+    local f = io.popen("amixer get " .. warg)
     local mixer = f:read("*all")
     f:close()