More power to extracts

[bwl21]

Request

Extracts should be able to

1. specify a transposition
2. specify which parts are rendered

Approach

  extract: {
    shift:"CD",     # similar to shift=
    range: [1-10]   # similar to %%clip
}

These parameters will be applied when retrieving the model by prepending the ABC-text.

[bwl21]

see also #212 which would allow more reuse of existing extracts