Implement async reader

Author: 999eagle

Cargo.toml

@@ -16,13 +16,16 @@ document-features = { version = "0.2", optional = true }
 encoding_rs = { version = "0.8", optional = true }
 serde = { version = "1.0", optional = true }
 memchr = "2.5"
+tokio = { version = "1.19", optional = true, default-features = false, features = ["io-util", "fs"] }
+async-recursion = { version = "1.0", optional = true }
 
 [dev-dependencies]
 criterion = "0.3"
 pretty_assertions = "1.2"
 regex = "1"
 serde = { version = "1.0", features = ["derive"] }
 serde-value = "0.7"
+tokio = { version = "1.19", default-features = false, features = ["macros", "rt-multi-thread"] }
 
 [[bench]]
 name = "bench"
@@ -94,6 +97,9 @@ serialize = ["serde"]
 ## Enables support for recognizing all [HTML 5 entities](https://dev.w3.org/html5/html-author/charref)
 escape-html = []
 
+## Enable support for asynchronous reading
+async = ["tokio", "async-recursion"]
+
 [package.metadata.docs.rs]
 all-features = true