feat(port): allow custom listener port

[dpritchett]

❤ THANKS FOR HELPING OUT :D

Proposed change

Add a slack_listener_port config parser to allow for overriding the default port of 3000 for the slack events listener.

Why? Heroku's docker-based setup insists on being able to inject a PORT variable, probably so their backend load balancers can more easily pack 'em in.

Types of changes

What types of changes is this pull request introducing to flottbot? Put an x in the boxes that apply

• Bugfix (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 not work as expected)

Checklist

You can fill this out after creating your PR. Put an x in the boxes that apply

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

I plan to show this in #flottbot slack and see if there's a good place to slide in some more relevant docs.

[dpritchett]

Having a few glitches with the test suite locally due to not running in Linux the way travis does and a checksum problem with the slack plugin. Please standby and I'll sort it.

[coveralls]

Pull Request Test Coverage Report for Build 742029b3a076c9caa1261bd3b07b42791ef5f3d0-PR-132

• 63 of 68 (92.65%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 42.71%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/outputs.go	0	1	0.0%
remote/slack/helper.go	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
remote/remote.go	2	0%

Totals
Change from base Build 0a738920d2087b9c0faa45fc96b9efe413fe883d:	0.4%
Covered Lines:	788
Relevant Lines:	1845

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💛 - Coveralls

[wass3r]

thank you for the contribution. looks good to me. just working on fixing our pipeline and this will get in. thanks for the patience.