Convert/replace specified files

Hi,

I think it would be nice to have a mode like this

``` bash
$ rake haml:convert_erbs -- app/views/foo/bar.html.erb
```

You only give ERB paths you want to convert/replace.

Another mode where confirmation is required could be nice

``` bash
$ rake haml:convert_erbs --confirm
Do you want to convert "foo/bar.html.erb" ? [Yn] Y
Do you want to convert "foo/baz.html.erb" ? [Yn] n
```
