feat: Put EvalFlag public

[tkanos]

In order to use flagr as a go package, EvalFlag should be public.

ref #298

Description

I put evalFlag public

Motivation and Context

As I work with flagr as a go package, to implement my own evaluator base on flagr solution. EvalFlag was the only function that needed to be public and was not. (see #298 )

How Has This Been Tested?

The test have been updated, but they don;t need to be changed

Types of changes

• Bug fix (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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added/updated tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #299 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #299   +/-   ##
======================================
  Coverage    81.8%   81.8%           
======================================
  Files          26      26           
  Lines        1533    1533           
======================================
  Hits         1254    1254           
  Misses        210     210           
  Partials       69      69

Impacted Files	Coverage Δ
pkg/handler/eval.go	77.45% <100%> (ø)	⬆️

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[zhouzhuojie]

LGTM