Change columns key to constants.

[k1LoW]

ref: #390

[github-actions]

Code Metrics Report

	main (ec726a6)	#393 (a4c1d24)	+/-
Coverage	67.1%	64.7%	-2.4%
Code to Test Ratio	1:0.5	1:0.5	-0.0
Test Execution Time	2m19s	3m43s	+1m24s

Details

  |                     | main (ec726a6) | #393 (a4c1d24) |  +/-   |
  |---------------------|----------------|----------------|--------|
- | Coverage            |          67.1% |          64.7% |  -2.4% |
  |   Files             |             33 |             33 |      0 |
  |   Lines             |           3175 |           3175 |      0 |
- |   Covered           |           2129 |           2054 |    -75 |
- | Code to Test Ratio  |          1:0.5 |          1:0.5 |   -0.0 |
  |   Code              |           8268 |           8269 |     +1 |
  |   Test              |           3887 |           3887 |      0 |
- | Test Execution Time |          2m19s |          3m43s | +1m24s |

Code coverage of files in pull request scope (63.0% → 63.0%)

Files	Coverage	+/-
output/md/md.go	80.3%	0.0%
schema/schema.go	29.0%	0.0%

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Reported by octocov