Add option to specify base source directory

Hi,

I added a option to configure base_dir for template sources.

For example, directory like this:

```
source
  ├── index.slim
  └── ng-templates
        └── logout.slim
```

index.slim:

```slim
== angular_include_templates 'ng-templates'
```

The path filled in `id` attribute would be related from `source` directory by default:

```html
<script type="text/ng-template" id="ng-templates/logout.html">
  <a href="/logout">Logout</a>
</script>
```

With this option:

```rb
activate :angular_templates, base_dir: 'source/ng-templates'
```

index.slim:

```slim
== angular_include_templates
```

The path would be related from the path specified with `:base_dir` option

```html
<script type="text/ng-template" id="logout.html">
  <a href="/logout">Logout</a>
</script>
```