Enhancement for mixin

The `=` operator supports whitespace after it
```
= var
```
but the call operator `+` does not
```
+ mixin
```
-> error

It would be visually better if the `+` also allows whitespace after it to align it with `=` before it, especially if using 2 space indent.
```
= var
+ mixin
  block
```

instead of

```
= var
+mixin
  block
```

Also enable support for module function call as mixin so we can do this (using jinja)

mymacro.html:
```jinja
{% macro wrap() %}
<div>{{ caller() }}</div>
{% endmacro %}
```

template.jade
```jade
- import 'mymacro.html' as mymacro
+ mymacro.wrap
  | content
```