Add support for docker context

[kaos14]

This will add support for docker context to display the current docker context name in the prompt.

Docker context is the currenlty selected node, or cluster on which all subsequent docker commands will be executed. It is like being logged into a different host, but only for docker commands.

Quering for docker context name (docker context inspect -f '{{.Name}}') can be slow if the context is other than default, as it involves communication with the target host over the network. This adds noticeable delay to prompt. When using the default context the delay is unnoticeable.