Add i18n support

[tirinnovo]

This commit is to allow adding supported or custom translations, e.g.

import Gantt from 'frappe-gantt';

const gantt = new Gantt('#gantt', tasks, {
  language: 'it', // or 'it-IT'
});

You can also create your own translations:

import Gantt from 'frappe-gantt';

const myEoLocale = {
  Mode: 'Modo',
  Today: 'Hodiaŭ',
  Year: 'Jaro',
  // ...
};

const gantt = new Gantt('#gantt', tasks, {
  language: 'eo',
  locales: {
    'eo': myEoLocale,
  },
});