Configurable font support

Use fontconfig to load fonts. Fonts can be added to the `fonts` map in
the config, and referenced from individual widgets' configurations for
customizability.

Fonts default to values "mono" and "sans" if nothing is set.

Cargo.toml

@@ -22,7 +22,7 @@ pulseaudio-widget = ["libpulse-binding"]
 [dependencies]
 smithay-client-toolkit = "0.6.4"
 byteorder = "1.3"
-rusttype = "0.8"
+rusttype = "^0.9"
 chrono = "0.4"
 nix = "0.16"
 memmap = "0.7"
@@ -43,3 +43,4 @@ serde_yaml = { version = "0.8", optional = true }
 
 libpulse-binding = { version = "2.15", optional = true }
 alsa = { version = "0.4.0", optional = true }
+fontconfig = "0.2.0"