add generic section definition and optionnal activation

[mickours]

Allows users to add additional sections to take advantage of the hooks and provide an activation mechanism using CLI. For example:

# in recipe
setup:
...
my_section:
...
export:

# in steps
my_step:
  - on_my_section_init:
   ...

# using CLI to run only setup and my_section
kameleon build my_recipe --section [setup, my_section]

[npf]

+1

[mickours]

An other way to do this is the use of YAML tags like in this project:
http://vagga.readthedocs.org/en/latest/config_overview.html#commands

We can use these tags to select a subpart of the recipe. For example I tag some steps with !my_steps and than use an option to execute only my tag or the other way around to exclude some tags like:

kameleon my_recipe.yaml --only-tagged  my_steps
#or
kameleon my_recipe.yaml --exclude-tagged my_steps

What do you think?

[npf]

Looks nice.