Lambda function for out_of_vocabulary_token_option

[keshprad]

This is a solution for lambda function feature request in #23

1. Created out_of_vocabulary_handler
   • Added ability to handle lambda function
     • the lambda function gets token_og_case, which has the original casing
   • I realized that the current release is implicitly using .lower() if an invalid option for out_of_vocabulary_token_option is passed (eg: out_of_vocabulary_token_option = 'upper')
     • This is due to L128.
     • I'm now passing the token with original casing to the handler function. If an invalid option is passed, I will use .title()
2. Added test case for lambda function.
3. Updated README for out_of_vocab options

[keshprad]

PR isn't ready to merge yet. I'm unsure how to address this, but would be a quick fix after we discuss