Should be able to configure the implementation directory

**Current behavior**
The implementation directory is `step_implementations`

**Proposed behavior**
The user should be able to configure the property

**Version**
```
Gauge version: 0.9.9.nightly-2018-05-29
Commit Hash: 1a15b83

Plugins
-------
ruby (0.5.1.nightly-2018-05-29)
```