Add static comment helpers

[jg-rp]

This PR adds BoundTemplate.comments() and BoundTemplate.docs() for statically retrieving a template's {% comment %}, {% # inline comment %} and {% doc %} nodes.

from liquid import parse

source = """\
{% doc %}
    some doc comment
{% enddoc %}

Hello!

{% comment %}
    some comment
{% endcomment %}

{% if false %}
    {% # an inline comment %}
{% endif %}
"""

template = parse(source)
print([node.text for node in template.comments()])
print([node.text for node in template.docs()])

Output

['\n    some comment\n', 'an inline comment']
['\n    some doc comment\n']

Fixes #199
Fixes #196.