Support arg+ syntax for argument capture

[johnrengelman]

Proposed change

Allow a rule to capture the remaining arguments by adding a + to the argument name in the rule.
Variable argument capture can only be activated on a single argument at the end of the argument list. It cannot be combined with optional arguments.

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)

[coveralls]

Pull Request Test Coverage Report for Build 229

• 0 of 26 (0.0%) changed or added relevant lines in 1 file are covered.
• 431 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-21.9%) to 17.893%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/matcher.go	0	26	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/prommetric.go	2	0.0%
core/configure.go	113	0.0%
core/matcher.go	316	0.0%

Totals
Change from base Build 227:	-21.9%
Covered Lines:	360
Relevant Lines:	2012

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💛 - Coveralls

[wass3r]

@johnrengelman thanks for the contribution! Just one quick fix. I have tested otherwise and seems good to me. If you're up to it, would love to see this mentioned in docs. I created an issue for it at target/flottbot-docs#34.

[wass3r]

Super quick fix here. This trips up when length of rule.Args is zero. A fix here should get us past the tests.

[wass3r]

Thanks @johnrengelman for the idea and majority of the work. We finished it out in #116