Add config option for faux scrollback

Some people have complained about the behavior of faux scrollback inside
of vim/tmux, however from what I can tell, alacritty behaves the same
way as other terminal emulators that support faux scrollback.

However there are a lot of terminal emulators that do not support faux
scrollback at all, which leads to people complaining about unusual
scroll behavior.

This is my proposal on how to solve this issue, by giving people that do
not like the VTE-Style faux scrolling the option to opt-out.

src/config.rs

@@ -65,6 +65,10 @@ fn deserialize_duration_ms<'a, D>(deserializer: D) -> ::std::result::Result<Dura
 pub struct Mouse {
     pub double_click: ClickHandler,
     pub triple_click: ClickHandler,
+
+    /// Send up/down arrow when scrolling in alt screen buffer
+    #[serde(default="true_bool")]
+    pub faux_scrollback: bool,
 }
 
 impl Default for Mouse {
@@ -75,7 +79,8 @@ impl Default for Mouse {
             },
             triple_click: ClickHandler {
                 threshold: Duration::from_millis(300),
-            }
+            },
+            faux_scrollback: true,
         }
     }
 }

src/input.rs

@@ -391,7 +391,9 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
                 };
 
                 for _ in 0..(to_scroll.abs() as usize) {
-                    if self.ctx.terminal_mode().intersects(mode::TermMode::ALT_SCREEN) {
+                    if self.mouse_config.faux_scrollback &&
+                        self.ctx.terminal_mode().intersects(mode::TermMode::ALT_SCREEN)
+                    {
                         // Faux scrolling
                         if code == 64 {
                             // Scroll up one line
@@ -426,7 +428,9 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
                                 65
                             };
 
-                            if self.ctx.terminal_mode().intersects(mode::TermMode::ALT_SCREEN) {
+                            if self.mouse_config.faux_scrollback &&
+                                self.ctx.terminal_mode().intersects(mode::TermMode::ALT_SCREEN)
+                            {
                                 // Faux scrolling
                                 if button == 64 {
                                     // Scroll up one line