Allow REPL-style input/output

[yongrenjie]

When exec'ing the doctests we would have to strip away the first n characters. Probably this will be set with yet another option.

[yongrenjie]

from https://stackoverflow.com/a/25558325 (slightly updated for py3), the following allows a chunk of text to be directly executed as if in a REPL:

import sys
import code

infile = open('cmd.py')
def readcmd(prompt):
    line = infile.readline()
    if not line:
        sys.exit(0)

    print(prompt, end="")
    print(line.rstrip())
    return line.rstrip()

code.interact(readfunc=readcmd)

• it does mean that the user shouldn't put >>> at the start of every line.
• it probably also means that the input & output can be combined into one block instead of having the input be effectively duplicated