Added an initial proposal for the preupdate hook.

Created an initial proposal/direction for the preupdate hook. It is
heavily based on the update hook, with only minor modifications to that
code. This means that it entirely translate the original C API.

Cargo.toml

@@ -37,7 +37,7 @@ trace = ["libsqlite3-sys/min_sqlite_version_3_6_23"]
 bundled = ["libsqlite3-sys/bundled", "modern_sqlite"]
 buildtime_bindgen = ["libsqlite3-sys/buildtime_bindgen"]
 limits = []
-hooks = []
+hooks = ["libsqlite3-sys/preupdate_hook"]
 i128_blob = ["byteorder"]
 sqlcipher = ["libsqlite3-sys/sqlcipher"]
 unlock_notify = ["libsqlite3-sys/unlock_notify"]