[Refactor] Refactoring unitsData in progress: CUSpecifier renamed
[Refactor] Change property 'CU' within CUs to 'specifier' in unitsData.js and make correponding changes in content script
[Refactor/Broken-P1] Change unitsData format, and update (some of the) related code accordingly

Basiic CU selection and navigation works. Code for keyboard shortcuts needs to be udated to work with the new format.
[Data/Broken-P1 carry-forward] Add entry for urbandictionary.com in unitsData

Additionally, carry forward the issue caused in precious commit to be resolved later
[Refactor] Fix feature left broken in commit ec546be and other changes

- Fix broken broken feature (i.e. handing shortcuts specified unitsData
  that was left unfixed in commit ec546be
- Improve the way functions are stringified and destringified
- Change google search 'last year results' shortcut to use only selector
  (instead of the sequence of three, earlier)
- Change the terms "CU" to "MU" (main unit) and "miniUnit" to "miscUnit"
- Other minor improvements