Fix multi line command history

[robertoffmoura]

• Prevent extra lines from sticking to terminal after cycling through multi line commands in history
• Fix cursor position when cycling through multi line command history. Allow cursor to move between lines of a single multi line command
• Expose cursor position and add tests to check cursor position when cycling through multi line commands in command history

This fixes the following bugs:

1. When cycling through the command history, if one of the commands happens to be multi line, only the last line will be erased from the terminal after cycling away from that command. This means all the other lines of that multi line command will "stick" to the terminal instead of being erased.'
   Steps to reproduce:

• Type x = 1, press return
• Copy and paste the following two line command on the terminal, then press return.

  x = [1 2
  3 4]
  

• Press the UP arrow key twice.
• Verify that x = [1 2 is still displayed in the terminal, when it should have been erased.

2. Cycle through the command history and stop at a multi line command. Pressing the left arrow key only lets you move the cursor inside the last line. The lines before that are unreachable.

[dklilley]

CLA previously signed