Add ALSA widget (#18)

Add initial ALSA widget, and put both ALSA and PulseAudio behind feature flags.

Cargo.toml

@@ -8,7 +8,11 @@ edition = "2018"
 lto = "fat"
 
 [features]
+default = ["alsa-widget", "pulseaudio-widget"]
 damage_debug = []
+alsa-widget = ["alsa"]
+pulseaudio-widget = ["libpulse-binding"]
+
 
 [dependencies]
 smithay-client-toolkit = "0.6.4"
@@ -20,7 +24,6 @@ memmap = "0.7"
 os_pipe = "0.8"
 wayland-client = { version = "0.23", features = ["dlopen"] }
 wayland-protocols = { version = "0.23", features = ["client", "unstable_protocols"] }
-libpulse-binding = { version = "2.11"}
 dbus = "0.6"
 fuzzy-matcher = "0.2"
 lazy_static = "1.4"
@@ -30,3 +33,6 @@ shlex = "0.1"
 serde_json = "1.0"
 serde_yaml = "0.8"
 serde = { version = "1.0", features = ["derive"] }
+
+libpulse-binding = { version = "2.11", optional = true }
+alsa = { version = "0.3.0", optional = true }