Add keyword support to simple_tag and inclusion_tag

[dougn]

for a project I found it very useful to support named keyword args on simple and inclusion tags.

Example:

@register.simple_tag
def mytag(a, b=None, c=None):
    return {}

@register.inclusion_tag("example.html")
def inctag(data, **kwdargs):
    return kwdargs

{% mytag c="hello" a=3 %}
{% inctag var1=3 user=user foo="bar" %}

<!-- just a few of the errors detected as proper template errors -->
{% mytag c=5 %} <!-- template error! "%s takes at least %s non-keyword arguments (%s given)" -->
{% mytag 1 bad=4 %} <!-- template error! %s got an unexpected keyword argument '%s' -->
{% mytag 1 c=3 c=4 %} <!-- template error! %s got multiple values for keyword argument '%s' -->

contains complete error
Including a patch for this, sans the unit tests. jezdez will help with that.

[dougn]

initial patch with full support (but no unit tests)

[eopadoan]

FWIW, I had to wrap {% include %} tags with {% with %} tags sometimes, so I can see many use cases for the inclusion tag.

[Jacob]

Since we've standardized on this "=" syntax is seems to make sense for the simple tags to support this, too.

[Julien Phalip]

Closing as dupe of #13956, which as a patch that's pretty much ready to go.