Bug: fix running stdin

[caitlinelfring]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bug fix for stdin, which stopped working on v0.8.1.

What is the current behavior? (You can also link to an open issue here)
Fixes #85

What is the new behavior (if this is a feature change)?
Previous behavior:

$ echo "whitelist" | go run main.go --stdin
No violations found. Stay woke ✊

New behavior:

$ echo "whitelist" | go run main.go --stdin
/dev/stdin:1:0-9: `whitelist` may be insensitive, use `allowlist`, `inclusion list` instead (warning)
whitelist
^

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?)
No

Other information:

stdin stopped working in v0.8.1, related to changes in #78 which checked if the file is a text file in a different place. It was causing IsTextFileFromFilename to open stdin, which it saw as an empty file and would ignore it.

[codecov]

Codecov Report

Merging #86 (bbe3537) into main (e0c7d98) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   95.58%   95.60%   +0.01%     
==========================================
  Files          21       21              
  Lines         476      478       +2     
==========================================
+ Hits          455      457       +2     
  Misses         12       12              
  Partials        9        9              

Impacted Files	Coverage Δ
pkg/util/contenttype.go	100.00% <100.00%> (ø)

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