Refactor nomultiline and multiline mode difference

[tompng]

We don't need so much if-else branch for multiline mode and nomultiline mode.

This will internally allow multiline input in nomultiline mode. (#641)
Supporting multiline in nomultiline is needed to implement bracketed paste, pasting multiline text into nomultiline mode.

To manually test multiline input in singleline mode

Input multiline input to IRB, to append multiline history

irb(main):001* if 1
irb(main):002*   2
irb(main):003> end

Toggle singleline mode by calling Reline.readline '>> '

irb(main):004> Reline.readline '>> '
>> 

Press up arrow

>> if 1
  2
end

You can try this reproduction step in an older version of Reline, and can see a bug like this

irb(main):004> if 1          █