Split render_differential to rendering logic and differential rendering algorithm part

[tompng]

def render_differential contains both LineEditor's rendering logic and Algorithm part of differential rendering.

This pull request splits them into def render and def render_differential

def render
  LineEditor specific rendering logic part
  construct lines to be rendered to the screen from prompt, input and dialogs
  render_differential(new_lines, new_cursor_x, new_cursor_y)
end

def render_differential(lines, cursor_x, cursor_y)
  pure alogithm part of differential rendering
  independent from promt, input and dialogs
end

Now, render_differential([], 0, 0) can be reused from other method in LineEditor to clear the screen. def clear_rendered_lines is not needed anymore.

[ima1zumi]

LGTM!