Attribute wasm_import_module not recognised

[cgmcintyr]

Description

Building cwa-rs results in compilation error E06581 due to wasm import_module not being recognised.

Steps To Reproduce

Environment

cargo 1.33.0-nightly (2cf1f5dda 2018-12-11)
rustc 1.33.0-nightly (a8a2a887d 2018-12-16)

Command

cd cwa-rs && cargo build

Output

Compiling cwa v0.1.0 (/home/cgm/devel/wasm/cwa-rs)
error[E0658]: The attribute `wasm_import_module` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
 --> src/raw.rs:1:3
  |
1 | #[wasm_import_module = "cwa"]
  |   ^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(custom_attribute)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.
error: Could not compile `cwa`.

To learn more, run the command again with --verbose.

[cgmcintyr]

PR also addresses #1 (comment)