Generate backstop references by scenario

[franrou]

Update the make backstop-references command to allow filter by scenario name and only generate the references for the targeted scenario

make backstopjs-reference [scenario-name]

[Eduardo-Morales-Alberti]

@franrou could you add documentation on how to generate the reference?

[franrou]

@Eduardo-Morales-Alberti done

[jorgetutor]

@franrou could you please remark in the documentation that this extra parameter is optional and that will generate all without it?

[Eduardo-Morales-Alberti]

@jorgetutor Changes applied, could you review them?

[Eduardo-Morales-Alberti]

Pending to add the filter also to test cc @franrou

[franrou]

@Eduardo-Morales-Alberti Filter parameter was added to to the backstopjs-test command

Ready to review