Accept a String in opts.paths as a basepath.

If a `String` is supplied instead, it will become the basepath for default paths.

For example:

```javascript
bowerFiles({ paths: 'path/for/project' })
.pipe(gulp.dest('client/src/lib'));

/*
    {
        bowerDirectory: 'path/for/project/bower_components',
        bowerrc: 'path/for/project/.bowerrc',
        bowerJson: 'path/for/project/bower.json'
    }
*/
```