support for composite String in src property

This fix allows to use composite `String` in `src` property:

```js
bemdecl: {
    all: {
        src: 'templates/**/*.html:!templates/web-sites/**/*.html',
        dest: 'bemX',
        options: {
            root: 'path/to/root',
            includes: [ 'includes', 'templates' ],
            cut: 1
        }
    }
}
```
Delimeter depends on platform: `;` on Windows, `:` on Unix.