Fixes #7817 and #9456.

- The include tag now has a 'with' option to include to provide extra context
  vairables to the included template.

- The include tag now has an 'only' option to exclude the current context
  when rendering the included template.

- The with tag now accepts multiple variable assignments.

- The with, include and blocktrans tags now use a new keyword argument format
  for variable assignments (e.g. `{% with foo=1 bar=2 %}`).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14922 bcc190cf-cafb-0310-a4f2-bffc1f526a37