feat: support optional arguments for APPEND command #456

Job	Run time
Checking fmt and docs	33s
Clippy check	25s
Build and test (ubuntu-latest, nightly)	1m 11s
Build and test (ubuntu-latest, beta)	1m 3s
Build and test (ubuntu-latest, stable)	58s
Build and test (windows-latest, nightly)	2m 1s
Build and test (windows-latest, beta)	2m 17s
Build and test (windows-latest, stable)	2m 47s
Build and test (macOS-latest, nightly)	1m 9s
Build and test (macOS-latest, beta)	1m 10s
Build and test (macOS-latest, stable)	51s
	14m 25s

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