Calendar I18n: some parts of the calendar can't be internationalized

Some parts of the calendar kit can't be internationalized:

- [Here](https://github.com/akveo/nebular/blob/master/src/framework/theme/components/calendar-kit/components/calendar-header/calendar-header.component.ts#L21) we have `Today` word hardcoded in the template.
- Usage of Angular `DatePipe` [here](https://github.com/akveo/nebular/blob/master/src/framework/theme/components/calendar-kit/components/calendar-header/calendar-header.component.ts#L18) and [here](https://github.com/akveo/nebular/blob/master/src/framework/theme/components/calendar-kit/components/calendar-navigation/calendar-navigation.component.ts#L24) require Angular locale to be loaded, but can't be resolved another way, for example, when we're using another i18n library, which is not in Angular core.
- `getDate()` functionality is used in computations inside `calendar-kit`, also as exported for public use and may be overridden to provided proper i18n and this override may lead to errors.