Allow the usage of push streams

[szmarczak]

Push streams are disabled by default, but they can be enabled from now on.

Fixes #16

[szmarczak]

@pietermees If you could leave some feedback there, that would be great!

[coveralls]

Pull Request Test Coverage Report for Build 204

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 99.002%

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Files with Coverage Reduction	New Missed Lines	%
source/index.js	1	86.67%
<!--	Total:	1		-->

Totals
Change from base Build 193:	-0.7%
Covered Lines:	381
Relevant Lines:	382

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💛 - Coveralls