Gh 798 add csv reader

[alanakbik]

This PR adds another way for loading classification datasets, namely datasets that are in CSV format. You need to pass a column format (like in ColumnCorpus), which indicates which column(s) in the CSV holds the text and which field(s) the label(s).

corpus = CSVClassificationCorpus(
    # path to the data folder containing train / test / dev files
    data_folder='path/to/data',
    # indicates which columns are text and labels
    column_name_map={4: "text", 1: "label_topic", 2: "label_subtopic"},
    # if CSV has a header, you can skip it
    skip_header=True)

closes #798

[alanakbik]

👍

[yosipk]

👍