Support for use without class extension

It would be nice to offer an option to use this gem without actually adding methods to the `Regexp` class. Something along the lines of:

```
require 'regexp-examples/module'

RegexpExamples.examples(/.../)
RegexpExamples.random_example(/.../)
```

As opposed to the current, "default" behaviour:

```
require 'regexp-examples'

/.../.examples
/.../.random_example
```