add urlencode support to Config

[taybin]

I found this helpful for working around encode/databases#145

class SafeConfig(Config):
    """Subclass of starlette.config.Config, but it will urlencode
       values if requested."""
    def __call__(self,
                 key: str,
                 cast: type = None,
                 default: typing.Any = undefined,
                 encode: bool = False) -> typing.Any:
        return self.get(key, cast=cast, default=default, encode=encode)

    def get(self,
            key: str,
            cast: type = None,
            default: typing.Any = undefined,
            encode: bool = False) -> typing.Any:
        value = None
        if key in self.environ:
            value = self.environ[key]
        elif key in self.file_values:
            value = self.file_values[key]
        elif default is not undefined:
            value = default
        if value:
            if encode:
                value = quote(value)
            return self._perform_cast(key, value, cast)
        raise KeyError(f"Config '{key}' is missing, and has no default.")

I'd prefer to have the function arguments be def __call__(self, key, *, cast, default, encode), but that breaks from the superclass arguments. It seems safer though as all of them except key are optional.

[tomchristie]

Not a bad idea, but going to close this off as stale.
If we get other user feedback along these lines in the future perhaps we could reconsider if it's worth prioritising or not.

(Passing thought: we might consider if Starlette really needs it's own config parsing code or not.)