Preserve unmatched placeholders

[ArmorDarks]

As mentioned here https://github.com/alexei/sprintf.js/issues/8#issuecomment-152675119, it would be nice to have option to return unmatched placeholders as they are.

For example, when we have this:

sprintf('Test number %s')

instead of 'Test number undefined' return 'Test number %s'

And same for named placeholders:

sprintf('%(name)s number 1')

instead of error about non-existing placeholder return '%(name)s number 1'

This will make possible complex chaining, when you need to process strings by few modules.

So far can't tell for sure how should it be done.

I have few options:

• Global option during sprintf invocation:

  sprintf = require('sprintf-js').sprintf({
    preservePlaceholders: true
  })
  
  sprintf('%(name)s number 1')` -> `'%(name)s number 1'`
  

• Live switch in and switch off:

  sprintf.preservePlaceholders(true)
  
  sprintf('%(name)s number 1')` -> `'%(name)s number 1'`
  
  sprintf.preservePlaceholders(false)
  

• Special sprintfp function:

  sprintfp('%(name)s number 1')` -> `'%(name)s number 1'`
  

[jonyrock]

+1