Add a skip state to skip processes when they don't apply to the pipeline #5

Hi,

I think that fail and halt help when we need to apply all the operations, but sometimes we could skip a operation.

I'm just doing an if in the execute method to not run anything, but it would be nice if I could do something like:

before do
  skip "no need to run it" if !params["config_value"]
end

I can provide the code, just need to make sure it's useful for the composable_operations gem.

thanks man!