fix(includes): propagate silent mode from included Taskfiles to tasks

[trulede]

When merging a taskfile, retain the taskfile silent value, when set true, by applying that value to all merged tasks.

fixes #1319

Test case to follow.

[vmaerten]

The issue with this approch is slient: false does not work anymore :

  hello-not-silent:
    silent: false
    cmds:
      - echo "Hello from include not silent task"

I took a look back in time, and figure out it'll be hard to solve this issue without the scoping.
(FYI, I'm going open a new experiment soon about scoped taskfiles)

[trulede]

@vmaerten I added a commit which handles that by parsing YAML into a *bool. But if you are going to change the merging with scoped taskfiles then its not worth the bother here.

[vmaerten]

Good job!
I've just extract an helper