Handle pluralization better

[Nevon]

If you take a look at the standard gettext method calls, you have more than one key in one method call, in the case of pluralization. For example:

i18n.ngettext('apple', 'apples', num)

gettext-loader would only pick up on 'apple'.

Even if we add %d to the strings, it will still only care about the first argument, so while I would like to get an entry like:

msgid "%d apple"
msgid_plural "%d apples"
msgstr[0] ""
msgstr[1] ""

I will just get:

msgid "%d apple"
msgstr[0] ""
msgstr[1] ""

I'm not sure how to work around this, but it seems to me that it would be a decent start to differentiate between function calls for singular translation and pluralized translations, and in the case of pluralized translations calls, record both the first and the second argument.

Something like this:

exports default {
  gettext: key => {
    // ... translate
  },
  gettextn: (singular, plural, num) => {
    // ... translate
  }
}

With this config:

export default {
  methods: {
    singular: ['gettext'],
    plural: ['gettextn']
  }
}

import {gettext, gettextn} from 'i18n'

gettext('Foo bar')

gettextn('One child', '%d children', 3)

Would result in:

msgid "Foo bar"
msgstr ""

msgid "A child"
msgid_plural "%d children"
msgstr[0] ""
msgstr[1] ""