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/clock.rs

@@ -1,18 +1,21 @@
 use crate::color::Color;
-use crate::draw::{Font, ROBOTO_REGULAR};
-use crate::widget::{DrawContext, DrawReport, KeyState, ModifiersState, WaitContext, Widget};
+use crate::draw::Font;
+use crate::{
+    fonts::FontRef,
+    widget::{DrawContext, DrawReport, KeyState, ModifiersState, WaitContext, Widget},
+};
 
 use chrono::{Duration, NaiveDateTime, Timelike};
 
-pub struct Clock {
+pub struct Clock<'a> {
     cur_time: NaiveDateTime,
-    clock_cache: Font,
+    clock_cache: Font<'a>,
     size: f32,
 }
 
-impl Clock {
-    pub fn new(time: NaiveDateTime, size: f32) -> Box<Clock> {
-        let mut clock_cache = Font::new(&ROBOTO_REGULAR, size);
+impl<'a> Clock<'a> {
+    pub fn new(time: NaiveDateTime, font: FontRef, size: f32) -> Box<Clock> {
+        let mut clock_cache = Font::new(font, size);
         clock_cache.add_str_to_cache("0123456789:");
 
         Box::new(Clock {
@@ -23,7 +26,7 @@ impl Clock {
     }
 }
 
-impl Widget for Clock {
+impl<'a> Widget for Clock<'a> {
     fn wait(&mut self, ctx: &mut WaitContext) {
         let target = (self.cur_time + Duration::seconds(60))
             .with_second(0)