Store bangs relevance in plugin

[foo-dogsquared]

While the bangs database from Duckduckgo has the ranking stored in the r key, it is better to assume that all databases have no rankings. UX-wise, it makes sense to make our plugin store the rankings especially if #1 is implemented where there is no relevance stored in the database.

Possible implementations:

• Modify the bangs database to store the ranking.
  This is the last preference to implement. It can be slow and can be interrupting to the database.

• Store in a separate file.
  Maybe in a JSON file, RON file, or even a simple plain-text file similar to ~/.bash_history.