Tool/unused i18n token finder

[wlycdgr]

Adds a script to tools/ to aid in identifying unused i18n tokens.

• Have you followed the guidelines in CONTRIBUTING.md?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do?
• Does your submission pass tests?
• Did you lint your code prior to submission?

[zarembsky]

1. Ensure that i18n_results directory is created.
2. Remove unnecessary return statement in getFilepaths.
3. Add exceptions list to narrow down the list of tokens which should be tested manually. The exceptions should be grouped and commented.
4. Run lint and check for errors, other than CRLF and console. Example: line 114 misses semicolon.
5. Add execution time a la i18n-checker.js
6. Have mercy and rename it to unused-i18n-tokens-finder.js. "Possibly" is already in your PLEASE NOTE output.
7. Run i18n-checker.js. It wipes out i18n_results, and does not produce results itself, though it reports errors.
8. Think about using both tools for other projects which may use locales. Maybe it is worth to put these locale tools in a separate repo. Example: edge_build_tools in a separate repo, and to use them you just pull it as a sibling of ghostery-extension.

[wlycdgr]

Revisions in progress.

1. done (b20fd10)
2. done (80e0adb)
5. done (bab629a)
6. done (eb21b9f)