Add 'flip' filter

[flexferrum]

According to this pull request: pallets/jinja#906

Applies a filter with arguments flipped in reverse order.
This becomes useful with filters like map where you can't easily re-order
the arguments yourself.
Example 1:
Say you have a list of names and you want to prefix them all with "Name: "
you could do it like this:
.. sourcecode:: jinja
{{ ["Carl", "Jane"] | map("flip", "format", "Name: %s") | join(",") }}
This would give you back:
Name: Carl, Name: Jane
Example 2:
Say you have a User object like below:
.. sourcecode:: python
class User(object):
def init(self, name, surname, age):
self.name = name
self.surname = surname
self.age = age
user = User('john', 'doe', 31)
... and you want to extract a couple of attribute values from it as a list
and process that.
This is one way to do that:
.. sourcecode:: jinja
{{ ['surname', 'name']
| map("flip", "attr", user)
| map("capitalize")
| join(", ")
}}
This would give you back
Doe, John

[mattiasb]

Note that it's unsure if this will get merged upstream. :)

[flexferrum]

Thanks. :) In wrost case I hide it under the compatibility mode flag. :)