Support parameters for liquid include tags [take 2]

This is a resubmission of #876 due to the heavy changes that have been made to `master` since it was submitted.

This PR adds the ability to send parameters to includes:

```text
{% include foo.md bar="baz" %}
```

It can be used like this:

```text
# in _includes/foo.md
{{ include.bar }} # outputs "baz" (without the quotes)
```