Allow function as pattern value

I have a scenario where a regular expression or simple string matching is not sufficient for determining which files to instrument. I've added support to pass a function as the pattern parameter so that I am able to run more advanced logic to determine which files to instrument.

Usage would be: 
```javascript
blanket.options("filter", function(filename) {
    if (filename.length > 8 && process.env.NODE_ENV === 'test') {
        return true;
    }

    return false;
});
```