Generating Cests in subdirectories

First off, thanks for such a nice tool.

Generating Cest files was a pain. Now it should work fine both with the following:

```
./codecept generate:cest SimpleTest
```

and with:

```
./codecept generate:cest Not/So/Simple/But/Sometimes/Occures
```

Also simplifies the checks when generating

```
./codecept generate:cest OloloCest.php
```

But still provides the ability to set the extension for the test file (some crazy perks might want that =):

```
./codecept generate:cest Trololo.superduperextension
```