REPL should work with optional braces

[odersky]

I want to be able to write in the REPL

scala> if true then
  print(1)
  printl2)

and get "12" printed. But right now, it prints 1 immediately.

scala> if true then                                                                                                
     |   print(1)
1

To make this work, the REPL should continue to read indented code until it encounters a line that starts further to the left, or a blank line. Otherwise put, any line that has the same or larger indendation margin as the first line after the then should be seen as a follow-on.