path to image in url

There is a setting ` --implicit_base_url` that allows the following url format: 
```
http://localhost:8888/?url=image.jpg&w=300&h=300&mode=crop
```
Which is great, but what I want is the following:
```
http://localhost:8888/image.jpg?w=300&h=300&mode=crop
```
Is it possible to add an optional flag to pass the image path in the url? Would you accept a pull request and how would you see it done?

<b>P.S.</b> I understand that I can rewrite the urls internally with something like nginx, but more of interested of why it was chosen to set the image path only as a url param.
:+1: