Add Minilog.log(), to allow replacement of console

If you use an injected `console` object to log browser or Node.js activity, you can use Minilog instead: they have similar interfaces. Monolog provides a `log()` method, which proxies to `debug()`.

So for instance, the following snippet:

```js
function doThings(console) {
    if (problem) {
        console.error('problem');
        return;
    }
    console.log('no problem');
}
```

Works seamlessly with Minilog instead of `console`:

```js
var Minilog = require('minilog');
doThings(Minilog);
```

Closes #13