"aligned" prop util and common test

[levithomason]

See this comment in the Container PR for the background on this PR.

This PR adds a prop util and common test for the aligned prop. This prop breaks the pattern of other props as noted in the comments and doc blocks.

<i aligned='left' />          // <i class="left aligned" />
<i aligned='right' />         // <i class="right aligned" />
<i aligned='center' />        // <i class="center aligned" />
<i aligned='justified' />     // <i class="justified" />

To support the missing aligned class only when the value is justified we now have useAlignedProp in propUtils:

const { aligned } = this.props

const classes = cx(
  useAlignedProp(aligned)
)

In tests, we assure it was implemented correctly like so:

import Foo from 'src/Foo'

common.implementsAlignedProp(Foo)

I tested the util and common test on another component in developing this PR and I'm sure as I can be that it is correct.

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