Add i18n support

This PR adds the possibility to overwrite the English default Strings as requested in #6. The developer can pass an `i18n` object as an option mapping langage keys to alternative represenations (see `/lib/en.json` for english defaults). I didn't offer i18n support for strings that definetely seemed to be for developers only.

Also there is a new `usage` key which can be passed a readable stream providing an alternative usage template.

I used the `visualwidth` module to assure that it works for chinese character, too. However for chinese to fully work in adventure, `terminal-menu` has to be fixed (possibly with this https://github.com/substack/terminal-menu/pull/16)