added **kwargs to chat_post_message to allow more parameters

More parameters may include "icon_url", which is what I wanted to use, but there are more such as "unfurl_links" and "icon_emoji". This approach allows the method to seamlessly support more API parameters from Slack without explicitly parameterizing them. In fact it might even be a good idea to just skip username, parse, and link_names, and use the same **kwargs pass through with them.