Make `save_html()` an S3 generic method #700

Job	Run time
routine / routine	4m 2s
R-CMD-check / setup	20s
website / pkgdown	4m 10s
R-CMD-check / macOS-latest (4.3.1)	4m 36s
R-CMD-check / windows-latest (devel)	6m 4s
R-CMD-check / windows-latest (4.3.1)	12m 21s
R-CMD-check / windows-latest (4.1)	9m 52s
R-CMD-check / windows-latest (3.6)	7m 50s
R-CMD-check / ubuntu-20.04 (devel)	3m 5s
R-CMD-check / ubuntu-20.04 (4.3.1)	4m 7s
R-CMD-check / ubuntu-20.04 (4.2.3)	3m 14s
R-CMD-check / ubuntu-20.04 (4.1.3)	3m 48s
R-CMD-check / ubuntu-20.04 (4.0.5)	5m 16s
R-CMD-check / ubuntu-20.04 (3.6.3)	4m 52s
	1h 13m 37s

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