input: Add keybindings for horizontal scroll

* Add `h` and `Left` for scrolling to left
* Add `l` and `Right` for scrolling to right

Introduce UpdateLeftMark UnerInput event to be fired when the above keys
are pressed

src/core/ev_handler.rs

@@ -44,6 +44,7 @@ pub fn handle_event(
             display::draw_for_change(out, p, &mut um)?;
             p.upper_mark = um;
         }
+        Command::UserInput(InputEvent::UpdateLeftMark(lm)) => p.left_mark = lm,
         Command::UserInput(InputEvent::RestorePrompt) => {
             // Set the message to None and new messages to false as all messages have been shown
             p.message = None;

src/input/mod.rs

@@ -106,6 +106,8 @@ pub enum InputEvent {
     UpdateUpperMark(usize),
     /// `Ctrl+L`, inverts the line number display. Contains the new value.
     UpdateLineNumber(LineNumbers),
+    /// `Right`, `Left`, `h` or `l` was pressed
+    UpdateLeftMark(usize),
     /// A number key has been pressed. This inner value is stored as a `char`.
     /// The input loop will append this number to its `count` string variable
     Number(char),
@@ -440,6 +442,7 @@ impl InputClassifier for DefaultInputClassifier {
                 modifiers: KeyModifiers::CONTROL,
                 ..
             }) => Some(InputEvent::UpdateLineNumber(!ps.line_numbers)),
+
             // Quit.
             Event::Key(KeyEvent {
                 code: KeyCode::Char('q'),
@@ -451,6 +454,26 @@ impl InputClassifier for DefaultInputClassifier {
                 modifiers: KeyModifiers::CONTROL,
                 ..
             }) => Some(InputEvent::Exit),
+
+            // Horizontal scrolling
+            Event::Key(KeyEvent {
+                code: KeyCode::Char('h'),
+                modifiers: KeyModifiers::NONE,
+            })
+            | Event::Key(KeyEvent {
+                code: KeyCode::Left,
+                modifiers: KeyModifiers::NONE,
+            }) => Some(InputEvent::UpdateLeftMark(ps.left_mark.saturating_sub(1))),
+            Event::Key(KeyEvent {
+                code: KeyCode::Char('l'),
+                modifiers: KeyModifiers::NONE,
+            })
+            | Event::Key(KeyEvent {
+                code: KeyCode::Right,
+                modifiers: KeyModifiers::NONE,
+            }) => Some(InputEvent::UpdateLeftMark(ps.left_mark.saturating_add(1))),
+
+            // Search
             #[cfg(feature = "search")]
             Event::Key(KeyEvent {
                 code: KeyCode::Char('/'),