WASM support

[bonsairobo]

I'm hitting this error when trying to use test-log from a WASM build:

panicked at library/std/src/sys/pal/wasm/../unsupported/time.rs:31:9:
        time not implemented on this platform

[teohhanhui]

Stacking also doesn't work:

#[wasm_bindgen_test::wasm_bindgen_test]
#[test_log::test]
async fn it_works() {
}

results in:

error: async functions cannot be used for tests

[d-e-s-o]

I don't think anybody has worked on WASM support yet. If you have a working prototype, feel free to open a pull request and we can discuss.