runner: add task specific help command to --help

[ChenJesse]

Since we have released flag support, we now support:
taskrunner [task] --help which gives rich details on the task like
description, supported flags.

looks like

[Usage]
        taskrunner [task...]
        taskrunner [task] --help
        taskrunner [option]

[Options]
  -config string
    	Configuration file to use
  -describe
    	Describe all tasks
  -ignore-cache
    	Ignore task invalidation cache
  -list
    	List all tasks except those marked "Hidden"
  -listAll
    	List all tasks including those marked as "Hidden"
  -non-interactive
    	Non-interactive mode (only applies when running the default set of tasks)
  -watch
    	Run in watch mode (only applies when passing custom tasks)

[Example Usage]
        `taskrunner mytask`
        `taskrunner mytask ---help`
        `taskrunner mytask1 mytask2`
        `taskrunner mytask --mode=dev`
        `taskrunner --config ./customconfig.json mytask --mode=dev`
        `taskrunner --list`

[toniihsia]

:catjam: