Make `save_html()` an S3 generic method #701

Job	Run time
routine / routine	2m 40s
R-CMD-check / setup	25s
website / pkgdown	2m 55s
R-CMD-check / macOS-latest (4.3.1)	5m 29s
R-CMD-check / windows-latest (devel)	5m 30s
R-CMD-check / windows-latest (4.3.1)	11m 10s
R-CMD-check / windows-latest (4.1)	9m 18s
R-CMD-check / windows-latest (3.6)	7m 35s
R-CMD-check / ubuntu-20.04 (devel)	3m 10s
R-CMD-check / ubuntu-20.04 (4.3.1)	3m 4s
R-CMD-check / ubuntu-20.04 (4.2.3)	3m 46s
R-CMD-check / ubuntu-20.04 (4.1.3)	4m 29s
R-CMD-check / ubuntu-20.04 (4.0.5)	3m 37s
R-CMD-check / ubuntu-20.04 (3.6.3)	3m 22s
	1h 6m 30s

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