add onlyif && unless features

[Zarne]

Pull Request (PR) description

add onlyif and unless parameter.
for example, being able to download an archive if a command exit 0 or 1

i use some code from puppetlabs exec provider to add this features

[bastelfreak]

@Zarne thanks for the PR, can you please squash the commits together?

[Zarne]

squash done