Add warning when file input has no matches

[camdencheek]

Description of Changes

A common user error is pointing to the incorrect path with the file input. It's often not clear this is the issue because you just see no logs coming through.

This PR adds a warning message if, when first starting up, the file input plugin doesn't find any files that match its include patterns.

{
  "level": "warn",
  "timestamp": "2020-07-09T11:19:19.243-0400",
  "message": "no files match the configured include patterns",
  "plugin_id": "$.file",
  "plugin_type": "file_input",
  "include": [
    "./test/asdfasdf"
  ]
}

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)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   75.73%   75.74%   +0.01%     
==========================================
  Files          61       61              
  Lines        3593     3595       +2     
==========================================
+ Hits         2721     2723       +2     
  Misses        649      649              
  Partials      223      223              

Impacted Files	Coverage Δ
plugin/builtin/input/file/file.go	78.80% <100.00%> (+0.15%)	⬆️

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