Prettify action execution output in CLI

Add CLI options to "st2 execution get" for displaying execution result. By default, the output will print the status and result of the action execution as text. If given "-d" option, the command will print the full detail of the action execution in table format. If given "-k <key name>", the command will select the given key in the result dict and print the value. Dot notation for nested dicts is supported.