Disallow colon at start or end of attributes

[closes #1182]

This prevents things like:

```jade
option(value="level1", class: selected)
```

from compiling, but if you really wanted the old behavior, you just have to quote the attribute:

```jade
option(value="level1", "class:" selected)
```

```html
<option value="level1" class:="class:" selected="selected">Level One</option>
```

This is a breaking change, so part of the 2.0.0 track.