Support `mermaid` for `er.format:`

[k1LoW]

Fix #368

[github-actions[bot]]

Code Metrics Report

	main (00eaf91)	#415 (3ee4853)	+/-
Coverage	67.2%	65.0%	-2.2%
Code to Test Ratio	1:0.5	1:0.5	+0.0
Test Execution Time	3m16s	2m30s	-46s

Details

``` diff | | main (00eaf91) | #415 (3ee4853) | +/- | |---------------------|----------------|----------------|-------| - | Coverage | 67.2% | 65.0% | -2.2% | | Files | 35 | 35 | 0 | | Lines | 3367 | 3389 | +22 | - | Covered | 2262 | 2202 | -60 | + | Code to Test Ratio | 1:0.5 | 1:0.5 | +0.0 | | Code | 8707 | 8750 | +43 | + | Test | 4081 | 4136 | +55 | + | Test Execution Time | 3m16s | 2m30s | -46s | ```

Code coverage of files in pull request scope (66.7% → 66.7%)

Files	Coverage	+/-
config/config.go	71.5%	+0.0%
output/md/md.go	80.1%	-0.2%
output/mermaid/mermaid.go	76.3%	-0.9%
schema/schema.go	28.4%	0.0%

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Reported by octocov

[github-actions[bot]]

Code Metrics Report

	main (00eaf91)	#415 (22b7069)	+/-
Coverage	67.2%	65.0%	-2.2%
Code to Test Ratio	1:0.5	1:0.5	+0.0
Test Execution Time	3m16s	3m23s	+7s

Details

``` diff | | main (00eaf91) | #415 (22b7069) | +/- | |---------------------|----------------|----------------|-------| - | Coverage | 67.2% | 65.0% | -2.2% | | Files | 35 | 35 | 0 | | Lines | 3367 | 3390 | +23 | - | Covered | 2262 | 2202 | -60 | + | Code to Test Ratio | 1:0.5 | 1:0.5 | +0.0 | | Code | 8707 | 8755 | +48 | + | Test | 4081 | 4136 | +55 | - | Test Execution Time | 3m16s | 3m23s | +7s | ```

Code coverage of files in pull request scope (64.1% → 64.1%)

Files	Coverage	+/-
config/config.go	71.5%	+0.0%
output/md/md.go	80.1%	-0.2%
output/mermaid/mermaid.go	76.3%	-0.9%
output/output.go	0.0%	0.0%
schema/schema.go	28.4%	0.0%

---

Reported by octocov