Add vim modeline to EDITOR_INPUT_INSTRUCTIONS #231

[sharils]

To solve #231, there are three options I can think of:

1. Open the temp file again after calling subprocess.run, which adds overhead to all other text editors
2. Ask individual vim users to add set backupcopy=yes to their vimrc file, which requires every vim users to set backupcopy which isn't default
3. Change vim setting by adding modeline to the EDITOR_INPUT_INSTRUCTIONS, which adds couple more characters to EDITOR_INPUT_INSTRUCTIONS

It seems that option 3 is a middle ground between overhead and user experience, thus this PR.

To test:

#!/usr/bin/env python3
import subprocess
import tempfile
with tempfile.NamedTemporaryFile() as f:
    f.write('\nvim:backupcopy=yes'.encode())
    f.flush()

    subprocess.call(['vim', f.name])

    f.seek(0)
    text = f.read().decode()
    print(text)

P.s. Instead of rename the file and write a new one, backupcopy=yes forces vim to make a copy of the file and overwrite the original one.

[sharils]

@ihabunek Would you mind reviewing this? Thanks.