Feature/Value aggregator

[jsconan]

Add a method to aggregate values from the data extractor.

# Build full names from multiple entries
extractor = ValueExtractor(["firstname", "lastname"], " ".join)
data = [
     {"firstname": "John", "lastname": "Smith"},
     {"firstname": "Jane", "lastname": "Doe"},
]
print([extractor.aggregate(row) for row in data]) # ["John Smith", "Jane Doe"]