names without dots, slashes or dashes are treated as "variable"

Template might contains something like this

```
[% BLOCK fill_message_content %]
    <br/>
[% END %]
...
[% PROCESS fill_message_content %]
```

so parser unable to resolve filename and dies with error

```
Fatal error: Unable to resolve file path:
promo/promo_message_box.inc @ pos 2087: "PROCESS fill_message_content"
```

this fix solves it.