[TIDOC-1524] 

This PR adds the ability to include images in Titanium API doc pages.

**To verify**:

1. Create new folder named 'images' in the titanium_mobile/apidoc folder.
2. Add an image to the folder.
3. In a apidoc .yml file, add an `<img/>` tag that reference, e.g.
```
<img src="images/my_image.png"/> 
```
4. Run deploy.sh and verify that specified image appears in published page.