Add syntax highlighting for async/await/try keyword

[tesuji]

Snippet:

async fn send_message(&self, content: &str) -> Result<MessageId, Error> {
    self.rate_limiter.acquire(1).await; // Acquire 1 permit of async semaphore
    
    let serialized_message = serializer.serialize(content);

    match writer.enqueue_message(serialized_message).await {
        Ok(id) => {
            // Sending the message was fully successful and the ownership had
            // had been passed. The permit now will get released when a response
            // is received.
            Ok(id)
        },
        Err(e) => {
            // Sending the message failed. Since the permit hasn't been used,
            // release it immediately.
            self.rate_limiter.release(1);
            Err(e)
        }
    }
}

After this change:

Closes #377

[tesuji]

@bors r? @ehuss

[tesuji]

I also sent a patch to github rust syntax repo. Hope the author not too busy
to merge it.

[ehuss]

Thanks, can you add some tests to syntax_test_misc.rs?

[tesuji]

I added a minor test, but I don't whether it is good enough.

[ehuss]

Seems good enough, thanks!

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push