Add `continue` and `break`.

Call them with no arguments to skip to the next iteration/stop the loop, respectively.

    for x in range 10 =>
      if x == 5 => break!
      print x
    # will only print numbers 0 through 4

This only works in `while` and `for` loops. Additionally, one cannot break out of a loop
from `finally`.

The loop will return whether it was not broken.

    ok = for x in range 10 => x
    # ok is True
    ok = while True => break!
    # ok is False
    ok =>
      # this is similar to an `else` branch in Python

Fixes #10. TODO: documentation.