Support run multiple tasks from a command

[ledongthuc]

First thing, thanks about cool task runner. I'm using robo in my projects and it works well.

I just asking do you have any plan in order to support multiple tasks in command. Such as:

robo build docker
robo asset build docker
robo build optimize deploy

(with build, docker, asset, optimize, deploy are tasks I defined in robo.yml)

I frequent use it to do some customize steps when developing. Example I just change a image and want to check it in development environment, I can quick change asset and run server again without build binary and rebuild docker.

Another solution's way to caching or watching changes of steps. Build binary step will watch the source code file changes, compose/optimize assets will watch image/css/js, docker build will watch

In case you have plan to support it but didn't have time to do that. I can help you create a pull request.