improve filterexpression regex

[oberhamsi]

templates with string escaping do not work:

        basic-syntax26': ['{{ "\"fred\"" }}', {}, "\"fred\""],
        {{ ""|add:"simon" }}`

another thing which doesnt work but which i dont plan on addinɡ too soon: translation strings

 __("translation string")__ 

filterexpression regex

• our regex https://github.com/oberhamsi/reinhardt/blob/master/lib/filterexpression.js#L35
• django regex https://github.com/django/django/blob/master/django/template/base.py#L484

[oberhamsi]

fixed for token.splitContents: django regex works identical in js 702ce6c