feat: support custom config

[43081j]

Allows you to specify your own babel options in package.json, for example.

{
  "postcss-lit": {
    "babelOptions": {
      "plugins": ["jsx"]
    }
  }
}

Fixes #44

[coveralls]

Pull Request Test Coverage Report for Build 4247075089

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 96.699%

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Files with Coverage Reduction	New Missed Lines	%
lib/parse.js	1	92.74%
lib/test/parse_test.js	1	99.49%
<!--	Total:	2		-->

Totals
Change from base Build 3622874084:	-0.02%
Covered Lines:	897
Relevant Lines:	905

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💛 - Coveralls