Fix split filter compatibility issues

[jg-rp]

This PR adds a new implementation of the split filter which resolves some compatibility issues when compared to Shopify/Liquid (the reference implementation).

The new split filter will be enabled by default when using liquid.future.Environment, and can optionally be registered with liquid.Environment for those that don't mind the behavioural change. Like this:

from liquid import Environment
from liquid.future.filters import split

env = Environment()
env.add_filter("split", split)

template = env.from_string('{% assign items = "" | split: "," %}{{items | size}}')
print(template.render())

Closes #134.