withIrregulars method for InflectorFactory

[cesarep]

Adds a new method for the in LanguageInflectorFactory along with a test for it.
It allows to specify an array of custom Irregular substitutions:

$irregulars = [
   ['singular1', 'plural1'],
   ['singular2', 'plural2'],
   [...]
];

InflectorFactory::create()->withIrregulars($irregulars)->build();

Should be very useful when a given ruleset is not getting some word right, the user could manually set the words it needs