Please add deep merge of templates

[Aliaksei-Tatarynchyk]

Current default merge strategy is just to override all the content. E.g. we have following setup:
playbook:

templates:
- templ2.tmpl
- templ1.tmpl
variables:
  var: value
  ...

templ1.tmpl:

root:
  child1:
    ...
  child2:
    ...

templ2.tmpl:

root:
  child3:
    ...

Result will be

root:
  child3:
     ...

It's completely not ok, because it makes templates hierarchy useless. Need to add deep merging of templates, so rendered result will be the following:

root:
  child1:
    ...
  child2:
    ...
  child3:
    ...