Configurable errors

[ArmorDarks]

It would be great to have possibility to switch off errors for empty placeholders in some cases

Especially in case of named placeholders, since, actually, they are the only one type of placeholders, which throw error:

Error: [sprintf] property 'val' does not exist

It's possible to make it configurable, just like I've described it in my comment here:

https://github.com/alexei/sprintf.js/issues/49#issuecomment-148887605

Or on sprontf() function invocation:

sprintf = require('sprintf-js').sprintf({
  errorOnUndefined: false
})