Better Tokenization

[abuchmueller]

• add support for additional tokenizers (moved to Add external tokenizer support #23)
• add support for custom stopwords
• handle usernames and hashtags automatically
• handle emojis
• n-gram support
• handle social media tags (moved to Handle social media tags #24)

[abuchmueller]

There are two possible scenarios for the implementation of external tokenizers:

1. a copy of pool_tweets() where instead of passing arguments to the quanteda tokenizer, an external tokenizer function is passed as an argument. This will be diffucult and tedious to test since there are many tokenizers.
2. a copy of the pool_tweets() function where instead of a finished document term matrix only the text/full corpus of the final document pool is returned. This is easier to implement and test but requires more effort from the user to work with.