Feature: Ability to provide barcode with checksum

[michalbundyra]

Added new option "providedChecksum" to indicate if the checksum has been
provided with the barcode text.

Resolves #13

We have now three "similar" options:

• withChecksum - whether or not the checksum is automatically added to the barcode
• withChecksumInText - whether or not the checksum is displayed in the textual representation
• providedChecksum - whether or not the checksum is provided with the barcode text

so the new option is about the input barcode text.
If it's provided validator checks if the provided checksum is valid.

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  • How will users use the new feature?
  • Base your feature on the develop branch, and submit against that branch.
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  • Add tests for the new feature.
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  • Add a CHANGELOG.md entry for the new feature.

[michalbundyra]

@christeredvartsen @Gemorroj @povils @silverslice would you be able to have a look on this PR and let me know if you are happy with that feature? I am pinging you, as you where interested in #13. Thanks a lot!