Implements skip stage

[rafaels88]

It introduces a new stage skip and solve the issue #1 .

Usage example:

  defmodule CreateUserPipeline do
    use Opus.Pipeline

    skip if: :user_exists?
    step :persist_user

    def user_exists?(_), do: true
  end

  CreateUserPipeline.call(%{}) # {:ok, :skipped}

[coveralls]

Pull Request Test Coverage Report for Build 60

• 5 of 5 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 53:	0.0%
Covered Lines:	151
Relevant Lines:	151

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💛 - Coveralls

[rafaels88]

Btw, sorry for the delay =/

[zorbash]

@rafaels88 Please make changes for

https://github.com/Zorbash/opus/pull/2#discussion_r202936935 https://github.com/Zorbash/opus/pull/2#discussion_r202937245

and it's good to merge :-)

[rafaels88]

@Zorbash Changed! Thanks for the review!

[zorbash]

@rafaels88 Thanks for contributing, I'll update the changelog and releases it as 0.6.0.