All files are excluded when exclude function contains ||

When exclude option is a function and it has operator ||, all files will be ignored.

For example, the following config will ignore all files:

```js
require('postcss-pxtorem')({
exclude: function exclude(file) {
return /node_modules/i.test(file) || false;
}
})
```

While the following code works properly:

```js
require('postcss-pxtorem')({
exclude: function exclude(file) {
return /node_modules/i.test(file);
}
})
```