Move hy.readline_helpers code into hy.completer

Move code from `hy.readline_helpers` into new `hy.completer.completion` context manager so we can "wrap" readline completion around REPL interaction like this:

```python
    with completion():
        if not hr:
            hr = HyREPL()

        hr.interact("{appname} {version}".format(
            appname=hy.__appname__,
            version=hy.__version__
            ))
```

Closes #226