Add Print Task

[fjtirado]

What would you like to be added:
A print task to be able to print message to the standard output

Using literal

do: 
   debug: Hello world!

Using expression (typically jq interpolation, since we should NOT support interpolation by ouselves)

do: 
   debug:  ${Hello \(.input.name)!}

Its up to every implementation decide how debug should be disabled (there is the possibility of adding the option of enable/disable at the header of the definition file, but I think this should be dynamic and enforcing that is kind of static)

Why is this needed:
It might be useful for debugging. Users are using custom sysout extensively, so I guess it will be good to include in the spec some way to print messages to console

[cdavernas]

@fjtirado Should we leave this issue opened, as we closed #859 ?