optimizing preprocessing by combined regex
- exported new method `prepare!` that can preprocess either a document or a corpus based on flags. It can also accept supplied patterns and words to be stripped from the document.
- modified all `remove_*` methods to use `prepare!`, except `remove_case!` and `remove_corrupt_utf8!` which remain as before.
- added deprecation warnings to `remove_*` methods.
- used regex in tokenizer instead of string split. makes it slightly faster.
- updated tests