Add support for "runtime" values #981

Job	Run time
Book	6s
Rustfmt	12s
Docs (rinja)	25s
Audit	17s
Docs (rinja_derive)	19s
Check order in Cargo.toml	4s
Docs (rinja_parser)	14s
DevSkim	31s
Jinja2-Assumptions	13s
Typos	25s
Test (ubuntu-latest, stable)	45s
MSRV	19s
Test (ubuntu-latest, beta)	40s
Package (examples/actix-web-app)	35s
Package (examples/axum-app)	31s
Test (macos-latest, stable)	42s
Package (examples/poem-app)	34s
Test (windows-latest, stable)	2m 26s
Package (examples/rocket-app)	33s
Package (examples/salvo-app)	34s
Package (examples/warp-app)	34s
Package (fuzzing)	31s
Package (rinja)	33s
Package (rinja_derive)	23s
Package (rinja_derive_standalone)	30s
Package (rinja_parser)	31s
Package (testing)	34s
Package (testing-alloc)	18s
Package (testing-no-std)	19s
Fuzz	0s
Cluster-Fuzz	0s
	14m 38s

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