allow TermPipeline configuration of indexer to be set not by property

[Craig Macdonald]

Discussed with @seanmacavaney and @tonellotto

pt.IterDictIndexer( 
stemmer : Union[NoneType, str, TerrierStemmer]
stopwords : Union[NoneType, TerrierStopwords]

from enum import Enum
class TerrierStemmer(Enum):
  none = 'none'
  porter = 'porter'
  weakporter = 'weak'

class TerrierStopwords(Enum):
  none = 'none'
  terrier = 'terrier'