Update weedle dependency to git dependency

As discussed in wasm-bindgen/weedle#52 (comment)

Author: RSSchermer

crates/webidl/Cargo.toml

@@ -21,7 +21,7 @@ proc-macro2 = "1.0"
 quote = '1.0'
 syn = { version = '1.0', features = ['full'] }
 wasm-bindgen-backend = { version = "=0.2.80", path = "../backend" }
-weedle = "0.12"
+weedle = { git = "https://github.com/rustwasm/weedle.git" }
 lazy_static = "1.0.2"
 sourcefile = "0.1"
 structopt = "0.3.9"