feat: add generic event filter (#536)

`WithFilter` lets you supply an event filter that will be invoked
before Bubble Tea processes a `tea.Msg`. The event filter can return
any `tea.Msg` which will then get handled by Bubble Tea instead of
the original event. If the event filter returns nil, the event
will be ignored and Bubble Tea will not process it.

As an example, this could be used to prevent a program from
shutting down if there are unsaved changes.

Based on the fantastic work by @aschey and supersedes #521.

Resolves #472.

options_test.go

@@ -35,6 +35,13 @@ func TestOptions(t *testing.T) {
 		}
 	})
 
+	t.Run("filter", func(t *testing.T) {
+		p := NewProgram(nil, WithFilter(func(_ Model, msg Msg) Msg { return msg }))
+		if p.filter == nil {
+			t.Errorf("expected filter to be set")
+		}
+	})
+
 	t.Run("startup options", func(t *testing.T) {
 		exercise := func(t *testing.T, opt ProgramOption, expect startupOptions) {
 			p := NewProgram(nil, opt)