This is now done in two steps:

- Parse the value into a sequence of atoms (literal of variable).
- Resolve that sequence into a string.

This fixes an issue when a variable is resolved differently in two
bindings.

For instance, take the following env file:

```
PORT=8000
URL=http://localhost:${PORT}
```

With `PORT` set to `1234` in the environment, the environment resulting
from `dotenv_load(override=False)` would be:

```
PORT=1234
URL=http://localhost:8000
```

This was inconsistent and is fixed by this commit.  The environment
would now be:

```
PORT=1234
URL=http://localhost:1234
```

with override, and

```
PORT=8000
URL=http://localhost:8000
```

without override.

The behavior of `load_dotenv` is unchanged and always assumes
`override=True`.

This mainly reorganizes the readme based on questions and feedback from
users on GitHub over the years:

- Getting Started: Short section which covers the main use case and
  doesn't go into details.
  - Pip command at the very beginning so that users are less likely to
    mistakenly install another package.
  - Basic application code to load the .env file into the environment.
  - Introduction to the syntax of .env files with a short example.
- Other common use cases:
  - Load configuration without altering the environment
  - Parse configuration as a stream
  - Load .env files in IPython
- Command-line Interface
- File format: Details about the syntax of .env files, previously
  scattered around.
- Related Projects: I'm not sure we really need that one but I guess we can keep
  it for now.
- Acknowledgements

Minor changes:

- I removed the "saythanks" link since it is dead.
- I removed the banner made in ASCII art since it read ".env" and not
  "python-dotenv", which I found distracting.  We could make another one
  but I don't have time right now.  It also saves the user some
  scrolling.

The default value for the `encoding` paramter of `load_dotenv` and
`dotenv_values` is now `"utf-8"` instead of `None` (which selected the
encoding based on the user's locale).  It is passed directly to
`io.open`.

The rationale for this change is that the encoding of a project file
like `.env` should not depend on the user's locale by default.  UTF-8
makes sense as the default encoding since it is also used for Python
source files.  The main drawback is that it departs from `open`'s
default value of `None` for the `encoding` parameter.

The default value of `None` was a source of confusion for some users.
The Flask and Docker Compose projects already use `encoding="utf-8"` to
enforce the use of UTF-8 and avoid that sort of confusion.

This is a breaking change but only for users with a non-UTF-8 locale and
non-UTF-8 characters in their .env files.