[idea] Non-exhaustive input mapping

[odinuv]

Create a new parameters for mappings setting - sth. like mode: permissive (with default being mode: strict - the current behaviour). In this mode, the mapping definion will only override the default mapping.
The advantage would be that e.g. to rename a single column, it would not be neccessary to enumerate all columns, etc.

Also possibly solves #56 and #26