Implement filterPath & filteringPathRegEx

This is useful if one is mocking for a client which appends
timestamps to the request path as GET parameter

Example: the search command of npm which is not adding a timestamp, but a startkey

```
/-/all/since?stale=update_after&startkey=1387902878592
```

https://github.com/robertkowalski/npm-registry-mock/issues/5