Add @composes rule support

[tivac]

Description

Implementing the idea from #633, but renamed to @composes because it's a concept already prevalent in modular-css so it seems like a good fit.

Motivation and Context

Fixes #633, which also explains why this is probably a good idea. It could be a terrible plan too, I dunno!

How Has This Been Tested?

Added tests & ensured code coverage looked good. I'm sure this first pass is missing featured but I'll get it there once I start using it for real.

Types of changes

• 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:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #635 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #635      +/-   ##
==========================================
+ Coverage      99%   99.02%   +0.01%     
==========================================
  Files          48       49       +1     
  Lines        1209     1230      +21     
  Branches      184      187       +3     
==========================================
+ Hits         1197     1218      +21     
  Misses         12       12

Impacted Files	Coverage Δ
packages/processor/plugins/composition.js	100% <100%> (ø)	⬆️
packages/processor/processor.js	100% <100%> (ø)	⬆️
packages/processor/plugins/graph-nodes.js	100% <100%> (ø)	⬆️
packages/processor/plugins/at-composes.js	100% <100%> (ø)

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[TravisBuddy]

Hey @tivac,
Something went wrong with the build.

TravisCI finished with status errored, which means the build failed because of something unrelated to the tests, such as a problem with a dependency or the build process itself.

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TravisBuddy Request Identifier: 54ac9690-9e2b-11e9-80b6-37a8a59c6ad9