fix: don't extract from common async chunks

[numical]

The problem

1. Create an entry point requiring two asynchronous chunks;
2. Name these asynchronous chunks;
3. Configure CommonsChunkPlugin to generate a common chunk explicitly for these two asynchronous chunks;
4. If the resultant commons chunk has embedded CSS, ExtractTextPlugin does not extract this.

See the added test case in this PR for the scenario.

This may be related to issues 120 and 459.

The cause
The generated async chunk has no parents and no entry point.

The fix
Replace all uses of chunk.isInitial() with chunk.isInitial() || chunk.parents.length === 0.

[codecov]

Codecov Report

Merging #508 into master will increase coverage by 0.05%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   90.38%   90.43%   +0.05%     
==========================================
  Files           6        6              
  Lines         364      366       +2     
  Branches       77       78       +1     
==========================================
+ Hits          329      331       +2     
  Misses         35       35

Impacted Files	Coverage Δ
index.js	91.11% <85.71%> (+0.07%)	⬆️

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[michael-ciniawsky]

Fixes #120 #453