Make python plugins optional

Wox crashes on Windows 8 when trying to load the Python plugins, i can't pinpoint the exact error, it happens in the Python.Runtime project when calling `Runtime.Py_Initialize();` in `runtime.cs` [right here](https://github.com/Rovak/Wox/blob/python-optional/Pythonnet.Runtime/runtime.cs?pr=%2Fqianlifeng%2FWox%2Fpull%2F51#L78)

This is more of a workaround instead of an actual fix for the Python.Runtime error

Other changes:
- Save StartWoxOnSystemStartup when changing checkbox
- Add new tab Plugins
- Add new option to optionally load python plugin