feat: support regex capture groups as arguments

[justmiles]

Proposed change

This change adds support for regex capture groups as input arguments (closes #10). It does so by taking advantage of named & numbered capture groups in the standard library.

This can be used in both "respond" and "hear" rules. Doing so adds argument support for "hear" rules, discussed in #204.

In the following example, the capture group named word is processed as an argument with the same name.

name: hear
active: true
hear: /can you say (?P<word>\S+) for me/
format_output: "I can say ${word} for you"

See the unit tests for more detailed examples.

Types of changes

• 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

• 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 2543700057

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 2498854726:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[wass3r]

thanks a ton. let us know if you're adding to docs, otherwise i'll make a story to do so.

[justmiles]

Here's an accompanying PR for documentation: target/flottbot-docs#57