scroll through file with J/K keys

src/core/Buffer.h

@@ -23,6 +23,8 @@ public:
     auto crbegin() const { return m_lines.crbegin(); }
     auto crend() const { return m_lines.crend(); }
 
+    int get_num_lines() { return m_lines.size(); }
+
 protected:
     LinesType m_lines;
     /* TODO: delete this */