feat: Allow specification of now --public in .drone.yml

[atomdmac]

Allows users of the Drone plugin to specify the --public option as they would when using the now CLI directory. To do this, simply add the option to the .drone.yml file:

my_build_step:
  image: lucap/drone-now
  public: true
  ..other options...

[lucaperret]

Hi @atomdmac, your feature request is quite interessant, could you complete the changes ?