volume: allow to pass multiple arguments to amixer (#44)

* Added option for a table to configure volume widget, allowing device specification also.

* Updated README.md with new configuration options for volume widget

* volume: allow passing a list of commandline arguments

README.md

@@ -447,8 +447,10 @@ Provides volume levels and state of requested mixers.
 Supported platforms: Linux (required tool: amixer), FreeBSD.
 
 - Arguments (per platform):
-  * Linux: takes the ALSA mixer control as an argument, i.e. `"Master"`,
-    optionally append the card ID or other options, i.e. `"PCM -c 0"`
+  * Linux: takes either a single argument containing the ALSA mixer control as
+    an argument, i.e. `"Master"`, or a table passed as command line arguments
+    to [amixer(1)](https://linux.die.net/man/1/amixer),
+    i.e `{"PCM", "-c", "0"}` or `{"Master", "-D", "pulse"}`.
   * FreeBSD: takes the mixer control as an argument, i.e. `"vol"`
 - Returns:
   * Linux: returns 1st value as the volume level and 2nd as the mute state of

widgets/volume_linux.lua

@@ -26,8 +26,17 @@ local function worker(format, warg)
         ["off"] = "♩"  -- "M"
     }
 
+    if type(warg) ~= "table" then
+      warg = { warg }
+    end
+
+    local cmd = "amixer -M get "
+    for _, arg in ipairs(warg) do
+      cmd = cmd .. " " .. helpers.shellquote(arg)
+    end
+
     -- Get mixer control contents
-    local f = io.popen("amixer -M get " .. helpers.shellquote(warg))
+    local f = io.popen(cmd)
     local mixer = f:read("*all")
     f:close()