Allow custom transformers to be used in the dataset DSL

[wstrinz]

PubliSci's RDFization classes all use the Data Cube vocabulary to represent its input. The DSL and server extension don't necessarily require this however, and could serve as a useful interface for accessing other conversion tools.

Currently, the data function of the dsl is used by specifying the input source, and various Data Cube or parser specific parameters, for example

data do
  source 'https://github.com/wstrinz/publisci/raw/master/spec/csv/bacon.csv'

  dimension 'producer', 'pricerange'
  measure 'chunkiness'

  option 'label_column', 'producer'
end

Resolving this issue would, for example, involve modifying dataset_dsl.rb so that something like

data do
  engine :bio-interchange
  source 'https://gist.github.com/wstrinz/7165201/raw/0f688ba7041e828d3336bb530aa7495d94022af1/example.gff3'
end

to convert a GFF3 file using BioInterchange would be possible.