Link stylesheet didn't work

Hi! This plugin is so usefull.

I'm trying to link a external CSS to my email template. Before this step the CSS was in HEAD section and this want a problem. But i want use SASS as pre-processor to make it simpler. So i need a external CSS file. But when i try to include the CSS file, inline-css print this in console:

``` javascript
{ Error: getaddrinfo ENOTFOUND main.css main.css:80
at errnoException (dns.js:28:10)
at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:79:26)
code: 'ENOTFOUND',
errno: 'ENOTFOUND',
syscall: 'getaddrinfo',
hostname: 'main.css',
host: 'main.css',
port: 80,
response: undefined }
```

This is the code:

``` javascript
const fs = require('fs');
const inlineCss = require('inline-css');

fs.readFile('entry.html', 'utf8', (err, data) => {
inlineCss(data, { url: './', removeHtmlSelectors: true }).then(html => {
fs.writeFile('index.html', html);
}).catch(function(reason) {
console.log(reason);
});
});
```

And this is the HTML:

``` html

...

```

I have also tried '/', '.'.

I can't understand where is the problem. In the example i link the file in the same way.

Thanks in advance!