Add 'renderOrder' property to schema to decide in which order components render

[micaww]

This allows you to specify a renderOrder property in the _component or _componentList property of a layout or component's schema.yml to determine the order in which components get rendered.

All component or component-lists that don't specify this property will default to 1.

Ex:

col1:
   _componentList:
      renderOrder: 1
      include:
         - some-component

col2:
   _component:
      renderOrder: 2
      include:
         - some-component

[coveralls]

Pull Request Test Coverage Report for Build 2446

• 76 of 76 (100.0%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 2440:	0.0%
Covered Lines:	4249
Relevant Lines:	4249

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💛 - Coveralls

[olsonpm]

Whoops, this PR also needs to add documentation for the new schema property. And i'm not sure, but this seems like the second schema property that affects behavior outside kiln, the other being version. I note this because the kiln docs say the schema is

the main connective tissue between components and Kiln.

so the clay team may want to keep that in mind with docs regarding the schema file