Add option to define matching groups to RegexpTagger

[alanakbik]

Currently, the RegexpTagger only matches the full given regular expression in a sentence. Sometimes, you may want to match only a part of the given expression instead.

This PR extends the RegexpTagger such that you can optionally specify a matching group, i.e.:

from flair.data import Sentence
from flair.models import RegexpTagger

# Define a RegexpTagger with two patterns
tagger = RegexpTagger(
    mapping=[
        # The first pattern matches entire quotes (including quotation marks)
        (r'["„»]((?:(?=(\\?))\2.)*?)[”"“«]', "quote"),

        # The second pattern matches quotes, but without the quotation mark
        (r'["„»]((?:(?=(\\?))\2.)*?)[”"“«]', "quote_part", 1),
    ]
)

# tag example sentence
sentence = Sentence('He said: "this is interesting"')
tagger.predict(sentence)

# print the different matches
print(sentence.get_spans("quote_part"))
print(sentence.get_spans("quote"))