New keyword extraction methods

[andrewtavis]

Please use this issue to suggest other methods for keyword extraction that could be included in kwx. Suggestions would ideally include some of the following:

• A blogpost or other source where the method is applied for related NLP tasks
• A research paper that details the method and its potential applications
• Source code for the method in Python or another language

Estimates of the model's efficacy would also be appreciated so that a new good first issue can be made and prioritized.

Thanks for any suggestions!