Allow schemes to be read from an URL

I use this tool for a long time now. And before that I used Tomorrow Night for a long time. This is really a great tool and I wanted to thank you for it :-)

This is what my change is about:

I like to store my schema as a gist and it would be nice to generate the files by just passing a URL to the `base16` command:

```
> ./base16 https://awesome-schemes.com/my-scheme.yml
```

In the case you're curious, this is [my scheme](https://gist.github.com/railsbros-dirk/6901884) (it's based on Tomorrow Night).