[v2.0.1] cannot declare variable without leading dash

In `test/test.jade`, you have:

```jade
_test  =  [{abc: "lalal"}]
name = function() {}
```

..which now compiles to:

```html
<_test> =  [{abc: "lalal"}]</_test>
<name>= function() {}</name>
```

..in `pug v2.0.1`
