Add parameter to ensure mapping is only performed on canonical transcript

[jorainer]

Related to issue #123, add a parameter canonical to proteinToGenome or proteinToTranscript to perform the coordinate conversion only using the canonical transcript.

[jorainer]

Thinking it over - maybe an even easier solution would be to simply pre-filter the whole database before with

edb <- filter(edb, filter = ~ tx_is_canonical == 1)

and then do the conversion - that way we will use only the canonical transcripts - no need for an additional parameter. Only thing is to document that properly.