Django-tables2

- Dropped support for Django 1.8, 1.9 and 1.10.
- Add `extra_context` argument to `TemplateColumn` [509](https://github.com/jieter/django-tables2/pull/509) by [ad-m](https://github.com/ad-m)
- Remove unnecessary cast of record to `str` [514](https://github.com/jieter/django-tables2/pull/514), fixes [#511](https://github.com/jieter/django-tables2/issues/511)
- Use `django.test.TestCase` for all tests, and remove dependency on pytest and reorganized some tests [515](https://github.com/jieter/django-tables2/pull/515)
- Remove traces of django-haystack tests from the tests, there were no actual tests.

This is the last version supporting Django 1.8, 1.9 and 1.10. Django 1.8 is only supported until April 2018, so consider upgrading to Django 1.11!

- Added `tf` dictionary to `Column.attrs` with default values for the footer, so footers now have `class` attribute by default [501](https://github.com/jieter/django-tables2/pull/501) by [mpasternak](https://github.com/mpasternak)

- Added `as=varname` keyword argument to the `{% querystring %}` template tag,
fixes [481](https://github.com/jieter/django-tables2/issues/481)
- Updated the tutorial to reflect current state of Django a bit better.
- Used `OrderedDict` rather than `dict` as the parent for `utils.AttributeDict` to make the rendered html more consistent across python versions.
- Allow reading column `attrs` from a column's attribute, allowing easier reuse of custom column attributes (fixes [241](https://github.com/jieter/django-tables2/issues/241))
- `value` and `record` are optionally passed to the column attrs callables for data rows. [503](https://github.com/jieter/django-tables2/pull/503), fixes [#500](https://github.com/jieter/django-tables2/issues/500)

- Added a `row_counter` variable to the template context in `TemplateColumn` (fixes [448](https://github.com/jieter/django-tables2/issues/488))

- Do not fail if `orderable=False` is passed to `ManyToManyColumn()`

- Added `separator` argument to `ManyToManyColumn`.
- Allow `mark_safe()`'d strings from `ManyToManyColumn.tranform()`
- Disabled ordering on `ManyToManyColumns` by default.