Provide a way to query issues qith Liquid (Feature #665)

Once we have full liquid support (see #604) it is desirable to have access to about all objects from within the language with issues being the most important object type here.

I started working on an extension which utilizes a simple treetop grammar to parse a DSL for filter specification and to subsequently create the issues and make them available to the user.

The syntax currently looks like this:

{% query my_query %}
status o
author_id = 1
{% endquery %}

|_. ID |_. Subject |_. Status |
{% for issue in my_query.issues %}
| {{issue.id}} | {{issue.subject}} | {{issue.status.name}} |
{% endfor %}

You have {{my_query.count}} open issues.

This generates the following example output:

You have 2 open issues.

What is missing yet is full support for all attributes and more user-friendly input as it currently relies on the filter representation inside the query class which sometimes is not obvious.

Also missing are facilities for sorting and a performant way of slicing (i.e. without first instantiating all previous issues).