Add visual bell support

This commit adds support for a visual bell. Although the Handler in src/ansi.rs
warns "Hopefully this is never implemented", I wanted to give it a try. A new
config option is added, `visual_bell`, which sets the `duration` and `animation`
function of the visual bell. The default `duration` is 150 ms, and the default
`animation` is `EaseOutExpo`. To disable the visual bell, set its duration to 0.

The visual bell is modeled by VisualBell in src/term/mod.rs. It has a method to
ring the bell, `ring`, and another method, `intensity`. Both return the
"intensity" of the bell, which ramps down from 1.0 to 0.0 at a rate set by
`duration` and `animation`.

Whether or not the Processor waits for events is now configurable in order to
allow for smooth drawing of the visual bell.

1 files changed, 26 insertions, 0 deletions

diff --git a/alacritty.yml b/alacritty.yml
index 9a3a3711..28ddf365 100644
--- a/alacritty.yml
+++ b/alacritty.yml
@@ -118,6 +118,32 @@ colors:
 #     cyan:    '0x93a1a1'
 #     white:   '0xfdf6e3'
 
+# Visual Bell
+#
+# Any time the BEL code is received, Alacritty "rings" the visual bell. Once
+# rung, the terminal background will be set to white and transition back to the
+# default background color. You can control the rate of this transition by
+# setting the `duration` property (represented in milliseconds). You can also
+# configure the transition function by setting the `animation` property.
+#
+# Possible values for `animation`
+# `Ease`
+# `EaseOut`
+# `EaseOutSine`
+# `EaseOutQuad`
+# `EaseOutCubic`
+# `EaseOutQuart`
+# `EaseOutQuint`
+# `EaseOutExpo`
+# `EaseOutCirc`
+# `Linear`
+#
+# To completely disable the visual bell, set its duration to 0.
+#
+#visual_bell:
+#  animation: EaseOutExpo
+#  duration: 150
+
 # Key bindings
 #
 # Each binding is defined as an object with some properties. Most of the