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.

src/widgets/audio/pulse_sound.rs

@@ -1,7 +1,10 @@
 use crate::cmd::Cmd;
 use crate::color::Color;
 use crate::widget::WaitContext;
-use crate::widgets::bar_widget::{BarWidget, BarWidgetImpl};
+use crate::{
+    fonts::FontRef,
+    widgets::bar_widget::{BarWidget, BarWidgetImpl},
+};
 
 use std::cell::RefCell;
 use std::collections::HashMap;
@@ -484,11 +487,12 @@ pub struct PulseAudio {
 
 impl PulseAudio {
     pub fn new(
+        font: FontRef,
         font_size: f32,
         length: u32,
         sender: Sender<Cmd>,
     ) -> Result<Box<BarWidget>, ::std::io::Error> {
-        BarWidget::new(font_size, length, move |dirty| {
+        BarWidget::new(font, font_size, length, move |dirty| {
             let device = PulseAudioSoundDevice::new(move || {
                 *dirty.lock().unwrap() = true;
                 sender.send(Cmd::Draw).unwrap();