Fix package name

The mismatch between the repo name and the package name means it's awkward to import w/  ``go get``. This moves ``config.go`` into a directory named ``config``.

This way users can simply run:

```
go get github.com/nelhage/go.config
```

And then in their code:

```go
import (
        "github.com/nelhage/go.config/config"
)

// ...

config.ParseConfig(etc)
```

As it stands without this patch, users would be required to rename the package:

```go
import (
        config "github.com/nelhage/go.config"
)

config.ParseConfig(etc)
```