Example problematic file:

```bash
hello=hi
greetings=${hello}
goodbye=bye
greetings=${goodbye}
```

It would result in `greetings` being associated with the empty string
instead of `"bye"`.

The problem came from the fact that bindings were converted to a dict,
and so deduplicated by key, before being interpolated.  The dict would
be `{"hello": "hi", "greetings": "${goodbye}", "goodbye": "bye"}` in the
earlier example, which shows why interpolation wouldn't work: `goodbye`
would not be defined when `greetings` was interpolated.

This commit fixes that by passing all values in order, even if there are
duplicated keys.

While `typing` was added in 3.5.0, `typing.Text` was only added in
3.5.2, and so causes an `AttributeError`, not an `ImportError`
  exception.

#256
#258

- Add `--export` option to `set` to make it prepend the binding with
  `export`.
- Make `set` command create the `.env` file in the current directory if
  no `.env` file was found.

If a value is unquoted and has two or more adjacent spaces (like in `a=b
c`), the parser would detect an error.  This commit fixes that.

Tabs and other whitespace characters are now also considered like space
characters in this case and I added relevant test cases.