modify htmldiv to support Julia expressions and flexgrid #598

Job	Run time
build	1m 56s
Julia 1.6 - ubuntu-latest - x64	2m 42s
Julia 1.6 - macOS-latest - x64	3m 14s
Julia 1.6 - windows-latest - x64	6m 50s
Julia 1.7 - ubuntu-latest - x64	2m 37s
Julia 1.7 - macOS-latest - x64	4m 43s
Julia 1.7 - windows-latest - x64	3m 20s
Julia nightly - ubuntu-latest - x64	3m 56s
Julia nightly - macOS-latest - x64	5m 41s
Julia nightly - windows-latest - x64	4m 48s
	39m 47s

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