Test webp support for lossy/lossless/alpha/animation compression

Added test for all webp compressions, lossy/lossless/alpha/animation. All options in webpFeature array has to return true for enabling webp in browser.

Default usage, full feature check
```javascript
window.slimmage = { 
	webpFeature: [ "lossy", "lossless", "alpha", "animation" ] // full webp support
}
```
Basic feature check, "lossy"
```javascript
window.slimmage = { 
	webpFeature: [ "lossy"] // basic webp support
}
```

credits and more information at: https://developers.google.com/speed/webp/faq#how_can_i_detect_browser_support_using_javascript