Add support for Cloudwatch Composite Alarms

[alxgrk]

Hey there,

as the title states, this PR intends to add support for composite Cloudwatch alarms. Without that change the following exception would be thrown:

.../lambda-cloudwatch-slack/node_modules/continuation-local-storage/context.js:55
    throw exception;
    ^

TypeError: Cannot read property 'MetricName' of undefined
    at handleCloudWatch (.../lambda-cloudwatch-slack/index.js:236:36)

Best regards,
Alex