feat(atomizer): Add blend modes

[carriemorrison]

Add missing blend modes:

https://developer.mozilla.org/en-US/docs/Web/CSS/background-blend-mode
https://developer.mozilla.org/en-US/docs/Web/CSS/mix-blend-mode
https://drafts.fxtf.org/compositing/#mix-blend-mode

We only support 1 background-blend-mode since atomic only supports 1 bg image

How Has This Been Tested?

Ran atomizer as per the testing docs:

➜  atomizer git:(blendModes) ./node_modules/.bin/atomizer examples/html/sample-2.html
Parsing file examples/html/sample-2.html for Atomic CSS classes

.Bgbm\(n\) {
  background-blend-mode: normal;
}
.foo .foo_Bgbm\(h\) {
  background-blend-mode: hue;
}
.bar .bar_Mbm\(s\) {
  mix-blend-mode: saturation;
}
.Mbm\(pl\) {
  mix-blend-mode: plus-lighter;
}

Also added a basic unit test.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

Review the following points, and put an x in all the boxes that apply. If you’re unsure about any of these, don’t hesitate to ask. We’re here to help!

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

---

I confirm that this contribution is made under a BSD license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[redonkulus]

I think if we are going to start adding tests for rules, we should create a new file called rules.js and start adding them there. Then it can be a 1 to 1 relationship with the existing rules.js file.

[carriemorrison]

yeah, I guess we need to decide if its beneficial as @src-code had mentioned we aren't really testing anything new here, just the values added...

[redonkulus]

We could probably automate the whole thing by looping through the entire rules.js file and running the test for each rule and creating a snapshot or something. I think all of this could be in a separate PR since we can do it for all rules at one time.

[carriemorrison]

Just added the 2 extra modes for https://drafts.fxtf.org/compositing/#mix-blend-mode