Add support for whitespace control in ftdetect

Docs: http://jinja.pocoo.org/docs/templates/#whitespace-control

Whitespace can be controlled before and after jinja tags by adding a `-` to the
appropriate jinja block marker.

```python
{% for i in [1,2,3] %}
  # ... SNIP ...
{% endfor %}
```

becomes

```python
{% for i in [1,2,3] -%}
  # ... SNIP ...
{%- endfor %}
```