[FEATURE] Add option to list all available extensions

At the moment, there is no option to list all available extensions.
The extensions are located in `pycvsanaly2/extensions`, but not every file is ready to use or throws some errors.

To identify which extension is usable and ready to activate i`ve introduced to new command line options:

* `cvsanaly2 --list-extensions`
* `cvsanaly2 -e` 

Both options do the same: List all available extensions.

With the current master, the code with output:

```bash
andygrunwald ~/Development/cvsanaly2 (master) $ cvsanaly2 --list-extensions
CommitsLOC, FileTypes, MessageWordsPrint, Metrics, Patches
```

PS: I`m very new to python. If there are some "python uncommon source / paradigm", please let me know. I will correct it.