stemmer 2.2.0

A NLTK-port to perform stemming in Dart / Flutter and reduce words to its base.

2.2.0 17/6/2021

• Added toLowerCase option for PorterStemmer and SnowballStemmer

2.1.0 18/5/2021

• Added SnowballStemmer.
• Improved README.md with example.

2.0.0 11/4/2021

• Added null-safety.

1.1.0 22/7/2019

• Added unit tests.
• Improved README.md.
• Added example usage.

1.0.0 21/7/2019

• Initial port of the NLTK PorterStemmer implementation.