Add include support

I detected this bug when I wanted to use *workers* in the plugin *atom-asciidoc-preview*
When i used a *Node* process with *child_process* i could not load *include*

I tried several solutions : 
- @Task.once with atom API (base on child_process)
- child_process (spawn and fork) with more configuration
- the main thread

In each test, include file was not loaded : 
```
asciidoctor: WARNING: <stdin>: line 155: include file not readable: include.adoc
```

After debugging, i saw that : 
```
var xhr = new XMLHttpRequest();
```

So i added : 
https://www.npmjs.org/package/xmlhttprequest

And i defined : 
```
  //Define overrideMimeType, not define by default in wrapper
  XMLHttpRequest.prototype.overrideMimeType = function() {};
```
Wich was not implemented