Add missing locale strings to y18n lookup

[nexdrew]

➕ Further testing revealed the following strings used in index.js were missing from y18n lookup:

"Path to JSON config file": "Path to JSON config file",
"Show help": "Show help",
"Show version number": "Show version number"

This PR fixes that.

Note that the first commit takes the naive approach, which results in using the en version for 'Show help' in this scenario:

var argv = require('yargs').help('h').locale('pirate').argv

The second commit addresses this by deferring the y18n lookup until the whole usage content is generated, instead of doing it when the option is defined.

Also note that this PR does not attempt to address the problem of updating locale strings before the desired locale is set (if it even is a problem):

var argv = require('yargs')
  .help('h')
  .updateStrings({
    'Show help': 'Use this here help desc instead' // applies to default locale en
  })
  .locale('pirate') // changes locale and thus discards previously updated strings
  .argv

See related discussion in PR #220.

[bcoe]

This is a bit weird, but like you say, also fairly elegant!

[nexdrew]

@bcoe Just so I'm clear, = ok to merge?

Do you want me to attempt a release once the es.json and fr.json PRs are merged, or is that something I should leave to you? I assume the general steps are something like:

1. update CHANGELOG.md in master
2. npm version
3. git push && git push --tags
4. npm publish --tag next
5. test yargs@next
6. npm dist-tag add yargs@x.y.z latest

[bcoe]

= ship it.

You've got the steps correct for the release, but let's wait for @zkat to update the Spanish translation before we release. I talked to her today she's on it :)

You can push the next release once we get things merged 👍 -- I think I'll have time for some yargs hacking this weekend, perhaps we can close a few more issues.