Enter `stringstool`, which can extract localizable strings from IB files.

Expected usage, `./Tools/stringstool extract --output-directory "Localized/en.lproj" "Nibs/*.xib".
Update English strings files with `stringstool`.
Add pjr TODOs.
Rename a misleading variable.
Create the output directory if it doesn't exist.
Don't add duplicate strings more than once.
Add a banner at the top of the extracted file.
Name match variables.
Allow keys to be ignored.
More correct explanation.
Revert "Update English strings files with `stringstool`."

This reverts commit 108beabba416da424cef951e9ed78eb0b8284982.
Merge branch 'master' into stringstool
Rename extract to extract-interface-builder + documentation.
Tell which version was used in the file banner.
Merge pull request #1222 from tiennou/stringstool

Stringstool