Add support for a {% debug %} extension tag.

[kevin-brown]

Cloned from #798 and rebased with latest master.

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This dumps the available variables, filters and tests. See Issue #174.
This is roughly equivalent to the Django Template Language {% debug %} tag, and typical usage like this:

{% debug %}

produces output like this:

{'context': {'_': <function _gettext_alias at 0x7f9ceabde488>,
            'csrf_token': <SimpleLazyObject: 'lfPE7al...q3bykS4txKfb3'>,
            'cycler': <class 'jinja2.utils.Cycler'>,
            ...
            'view': <polls.views_auth.Login object at 0x7f9cea2cbe48>},
'filters': ['abs', 'add', 'addslashes', 'attr', 'batch', 'bootstrap',
            'bootstrap_classes', 'bootstrap_horizontal',
            'bootstrap_inline', ... 'yesno'],
'tests': ['callable', 'checkbox_field', 'defined', 'divisibleby',
        'escaped', 'even', 'iterable', 'lower', 'mapping',
        'multiple_checkbox_field', ... 'string', 'undefined', 'upper']}

The output under Python2 and Python3 below 3.4 is slightly less compact, as pprint.format() did not introduce the 'compact' argument before 3.4.

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/cc @ShaheedHaque

[davidism]

Changed to rely on autoescape rather than calling escape unconditionally. This way it's usable outside HTML.

Rebased and added docs and changelog.