API/ENH: Styler template arguments
Usecase - I have a `Styler` subclass with a new template that I want to pass some arbitrary stuff into - which currently there isn't a great way to do (right now I'm overriding and re-implementing `.render()`).

Open to suggestions on the api, but I was thinking of adding `template_args` argument that would be passed to the template.

```python
from pandas.formats.style import Styler
from jinja2 import Template

class MyStyler(Styler):
    template = Template("""
        .... omitted
        {{ my_value_1 }}
        """)

MyStyler(df, template_args={'my_value_1': '...'}).render()
```

xref #11610 (master tracker)

pandas 0.19.2