Unify ignore(Regex)/only option into a function

As a proposal, introduce a predicate function which finally unifies `ignore`, `ignoreRegex` and `only` options.

example: Assuming the file is at `(Some other projects)/node_modules/(This project)/lib/index.js`

```
const path = require('path');
const projDir = path.resolve(module.filename, '..') + '/'';
const projNodeModulesDir = path.resolve(projDir, 'node_modules') + '/';

require('6to5/polyfill');
require('6to5/register')({
    filter: function(m, filename) {
        return filename.startsWith(projDir) && !filename.startsWith(projNodeModulesDir);
    }
    // Otherwise
    // ignore: '^' + ESCAPE_REGEX(projNodeModulesDir),
    // only  : '^' + ESCAPE_REGEX(projDir)
});
```


NOTE: I have not tested any compatibility, but it may work if I coded correctly.