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/alsa_sound.rs

@@ -1,6 +1,9 @@
 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 alsa::mixer::{Mixer, Selem, SelemChannelId, SelemId};
 
@@ -34,15 +37,20 @@ pub struct Alsa {
 }
 
 impl Alsa {
-    pub fn new(font_size: f32, length: u32) -> ::std::io::Result<Box<BarWidget>> {
+    pub fn new(font: FontRef, font_size: f32, length: u32) -> ::std::io::Result<Box<BarWidget>> {
         let mixer = Mixer::new(CARD_NAME, true)
             .map_err(|err| alsa_error_to_io_error("Failed to create ALSA mixer", &err))?;
         let mixer = Self { mixer };
         mixer.get_master_volume()?; // probing
-        Ok(BarWidget::new_simple(font_size, length, Box::new(mixer)))
+        Ok(BarWidget::new_simple(
+            font,
+            font_size,
+            length,
+            Box::new(mixer),
+        ))
     }
     #[inline]
-    pub fn get_master<'a>(&'a self) -> ::std::io::Result<Selem<'a>> {
+    pub fn get_master(&self) -> ::std::io::Result<Selem> {
         let master_id = SelemId::new(SELEM_NAME, SELEM_ID);
         match self.mixer.find_selem(&master_id) {
             Some(master) => Ok(master),

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();