Add support for chat.postMessage API method
Add support for chat.postMessage API method

Shiggiddie/dale,slackhq/node-slack-sdk,DLavin23/dale,slackapi/node-slack-sdk,peernohell/node-slack-client,cpww/dale,dshahin/node-slack-client,slackapi/node-slack-sdk,slackapi/node-slack-sdk,peernohell/node-slack-client,slackapi/node-slack-sdk,slackhq/node-slack-client