Add error for regex with no named groups

[camdencheek]

Description of Changes

A common error is to use a regex pattern without any named capture groups. This errors out in that case because it's almost guaranteed to be unintended.

Sample error message:

{
  "level": "error",
  "timestamp": "2020-07-09T10:10:32.855-0400",
  "message": "Failed to start carbon log agent",
  "error": {
    "description": "build pipeline: build plugins: no named capture groups in regex pattern",
    "suggestion": "use named capture groups like '^(?P<my_key>.*)$' to specify the key name for the parsed field",
    "details": {
      "plugin_id": "$.rp",
      "plugin_type": "regex_parser"
    }
  }
}

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #12 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   75.93%   76.02%   +0.09%     
==========================================
  Files          60       60              
  Lines        3560     3569       +9     
==========================================
+ Hits         2703     2713      +10     
  Misses        636      636              
+ Partials      221      220       -1     

Impacted Files	Coverage Δ
plugin/builtin/parser/regex.go	83.67% <100.00%> (+3.67%)	⬆️
plugin/builtin/input/tcp.go	84.72% <0.00%> (+1.39%)	⬆️

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