`build app` ignores name parameter

When running `python-appimage build app` the `-n` parameter gets ignored.

The function `execute()` at `commands/build/app.py` accepts a name parameter but it is not used.

By adding this check on line 154, the AppImage file is named correctly.
```python
if name is None:
    application_name = os.path.basename(appdir)
else:
    application_name = name
```

