Feature suggestion: command add --dry-run

[jneubert]

When I have an externally crafted mapping file, I currently cannot see a way to evaluate which of these mappings would "work" for wikidata - meaning: have a matching property value for the WDSOURCEPROPERTY column. The diff command does not provide this, the description of the not-yet-implemented check command seems to aim at a different use case.

An add --dry-run could provide the information where a WDTARGETPROPERTY would be added (plus warnings, where a different WDTARGETPROPERTY value is already attached, plus perhaps - with --verbose - messages where the actual property value is already in place).

In production use, the function would be very helpful to check input files in advance and avoid costly mistakes. I suppose, it could be useful in the development of wdmaper, too, because it can be run on live data again and again.

[nichtich]

already implemented (but not used as long as #24 has not been implemented)