added ability to add custom filters

[josephmancuso]

This PR adds the ability to add custom filters.

All we need to do is use the filter method on the view class:

def slug(item):
    return item.replace(' ', '-')

ViewClass.filter('slug', slug)

and then inside the template:

{{ app.name|slug }}

---

The real world example would be to just throw this inside a provider:

class UserModelProvider(ServiceProvider):
    ''' Binds the User model into the Service Container '''

    wsgi = False

    ...

    def boot(self, Request, ViewClass):
        ViewClass.filter('slug', self.slug)

    @staticmethod
    def slug(item):
        return item.replace(' ', '-')

and boom. You now have a filter inside all of your templates.

[coveralls]

Coverage increased (+0.02%) to 91.295% when pulling 76aae09 on add-jinja2-filters into b259858 on master.

[aisola]

Looks good to me!