set "app_root" from environment variables

Nobody has claimed this yet.

Hey,

I think it would be nice to be able to set the `app_root` from environment variables like this:
```yaml
app_root: "$GOPATH/src/$APP_PATH"
```

It should be pretty easy to implement this, there just would need to be a call to `os.ExpandEnv(c.AppRoot)` at the end of the [Load function](https://github.com/markbates/refresh/blob/master/refresh/config.go#L47). E.g.:

```go
func (c *Configuration) Load(path string) error {
data, err := ioutil.ReadFile(path)
if err != nil {
return err
}
err = yaml.Unmarshal(data, c)
if err != nil {
return err
}

c.AppRoot = os.ExpandEnv(c.AppRoot)

return err
}
```

Start at the Load function in refresh/config.go and inspect how YAML populates Configuration.AppRoot. Verify environment expansion for the app_root value, including the documented $GOPATH and $APP_PATH form; done means loading a configuration resolves those variables correctly.

Written by the indexing model from the issue text.