Add newline between components' styles

[geniusgordon]

#9

declare extractedCSS as an array, use .join('\n') when return

[mxstbr]

Could you add some tests for the new behaviour please? Thank you!

[zalmoxisus]

Seems like decreasing the perf almost twice for this operation: https://jsperf.com/string-concat-via-array.

UPDATE: Sorry it's just for stylelint, so perf doesn't matter :-D

[geniusgordon]

@mxstbr I don't know how to write tests for this, can't come up with an invalid testcase that produce the rule-non-nested-empty-line-before error. Any suggestions? Thank you!

[coveralls]

Coverage remained the same at 99.048% when pulling 7d8f1f5 on geniusgordon:master into 9d904c9 on styled-components:master.

[geniusgordon]

@zalmoxisus nice point though, never notice that perf issue

[mxstbr]

Sorry for the extremely slow response here, does this fix your issue? If so, why can't we add tests for it?

[geniusgordon]

It's ok now! I disabled rule-empty-line-before like the comment in #9 said

[EnoahNetzach]

I need this fix..
Sorry if I opened another PR, but I'll cherry-pick these commits, so attribution should remain untouched.