Implement CSS filter for image and advanced backgrounds

[yhatt]

This PR implements CSS filter support for Markdown image syntax. It applies to the inline image and the advanced backgrounds. User would be able to apply powerful filters easily.

Example

<!-- theme: gaia -->

![](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![blur](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![brightness](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![contrast](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![drop-shadow](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![grayscale](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![hue-rotate](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![invert](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![opacity](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![saturate](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![sepia](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)

Advanced backgrounds

<!-- theme: gaia -->

![bg](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg blur](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg brightness](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg contrast](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg drop-shadow](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg grayscale](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg hue-rotate](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg invert](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg opacity](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg saturate](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)
![bg sepia](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)

Arguments and mutiple filters

Of course each filter syntax can set the filter function arguments by filter:param(,param2...), and apply multiple filters with space-separated.

<!-- Arguments -->
![bg blur:20px](https://example.com/imageA.jpg)
![drop-shadow:0,5px,20px,#fff](https://example.com/imageB.jpg)

<!-- Multiple filters -->
![bg opacity:75% sepia](https://example.com/imageC.jpg)
![grayscale:.5 contrast:.75](https://example.com/imageD.jpg)

ToDo

• Add test case

[yhatt]

The latest stable Chrome has a rendering bug as same as the known issue. This issue will fix in Chrome beta (Chrome 67).

We are going to make an instance option to switch enabling/disabling this feature.