Add support for "runtime" values #976

Job	Run time
Audit	17s
Rustfmt	11s
Book	4s
DevSkim	29s
Docs (rinja)	25s
Typos	22s
Jinja2-Assumptions	15s
Check order in Cargo.toml	6s
Docs (rinja_derive)	23s
Docs (rinja_parser)	16s
MSRV	20s
Test (ubuntu-latest, stable)	54s
Test (ubuntu-latest, beta)	39s
Test (macos-latest, stable)	48s
Package (examples/actix-web-app)	42s
Package (examples/axum-app)	39s
Test (windows-latest, stable)	2m 0s
Package (examples/poem-app)	36s
Package (examples/rocket-app)	43s
Package (examples/salvo-app)	43s
Package (examples/warp-app)	43s
Package (fuzzing)	31s
Package (rinja)	37s
Package (rinja_derive)	28s
Package (rinja_derive_standalone)	37s
Package (rinja_parser)	30s
Package (testing)	42s
Package (testing-alloc)	27s
Package (testing-no-std)	26s
Fuzz	0s
Cluster-Fuzz	0s
	15m 53s

Provide feedback