Feature/basic plurals support

[defuncart]

A possible solution to resolve #14.

By depending upon intl package, we can use the Intl.plural method to determine the plural form for a given number.

Thus an input key of the form

pluralTest,Time left: %seconds$d |seconds:second:seconds:seconds:seconds:seconds|

would be parsed into the following method:

import 'package:intl/intl.dart';

String pluralTest({
    @required int seconds,
  }) {
    String text = _getText("pluralTest");
    if (seconds != null) {
      text = text.replaceAll("%seconds\$d", seconds.toString());
    }
    final _pluralsRegExp = RegExp(r"\|(.*?)\|");
    final plurals =
        _pluralsRegExp.firstMatch(text).group(0).replaceAll('|', '').split(':');
    if (plurals.length != 6) {
      return 'Issue with key pluralTest. Incorrect number of plurals.';
    }
    return Intl.plural(
      seconds,
      zero: text.replaceAll(_pluralsRegExp, plurals[0]),
      one: text.replaceAll(_pluralsRegExp, plurals[1]),
      two: text.replaceAll(_pluralsRegExp, plurals[2]),
      few: text.replaceAll(_pluralsRegExp, plurals[3]),
      many: text.replaceAll(_pluralsRegExp, plurals[4]),
      other: text.replaceAll(_pluralsRegExp, plurals[5]),
    );
  }

and can then be used as follows:

seconds	fr	en	es	de
0	Temps restant: 0 seconde	Time left: 0 seconds	Tiempo que queda: 0 segundos	Verbleibende Zeit: 0 Sekunden
1	Temps restant: 1 second	Time left: 1 second	Tiempo que queda: 1 segundo	Verbleibende Zeit: 1 Sekunde
2	Temps restant: 2 secondes	Time left: 2 seconds	Tiempo que queda: 2 segundos	Verbleibende Zeit: 2 Sekunden

(Note that I don't speak French or Spanish, so if I made a mistake, please let me know and I'll update the example).

Presently only one plural per key is supported. intl could also be used to add support for genders, however such advanced localizations may be better served by sticking to official ARB process.

For English and German, writing |seconds:second:seconds:seconds:seconds:seconds| and |Sekunden:Sekunde:Sekunden:Sekunden:Sekunden:Sekunden| is pretty overkill as it is always "seconds" except for one case. In the future some optimizations could be introduced so that not all six forms always need to be supplied.