Bug fix pattern suffix chars

While using the plugin I noticed that when I have a pattern that has suffix chars and no pattern after them the library throws an error:

TypeError: Cannot read property 'index' of undefined
      at Object.pattern.parse (/Users/.../Desktop/.../formatter.js/src/pattern.js:61:29)


For example I might have a pattern for a a VAT number that looks like this:
CHE-_ _ _._ _ _._ _ _ MWST

where the underscores are digits, so the pattern using formatter.js would be:

'pattern': 'CHE-{{999}}.{{999}}.{{999}} MWST'

which would throw an error when I open the page in a browser: 'Cannot read property 'index' of undefined' in line 61 of the  formatter.js/src/pattern.js file. The only way this works is if I added a pattern after the ' MWST' suffix chars. Something like this: CHE-{{999}}.{{999}}.{{999}} MWST{{*}}

but I don't want to do that because it will add an extra character to the end of the input (at the very least a space character) and then the input would not pass validation. I added a test so you can see a similar case fail and then I applied a fix.

Thank you for your time and for this awesome library :)