Add :attr_delims option

I want to use Slim with AngularJS but run into the problem.
I have this code in my template:
```
div ng-controller="myCtrl"
  p {{ page.content }}
```
And I get this error:
```
Slim::Parser::SyntaxError: Expected attribute
  STDIN, Line 2, Column 5
    p {{ page.content }}
       ^
```
That because Slim uses ```{``` and ```}``` symbols as attribute delimiters and there is no way to change it.
I've added new option ```:attr_delims``` so now I can set option like this:
```
Slim::Engine.set_default_options :attr_delims => {'(' => ')', '[' => ']'}
```
It resolves my problem.
I've added several tests to make sure that nothing broken.
Please, let me know what do you think about this change.
If you don't like it, I will appreciate any advise how to resolve this problem in another way.