option: right click ends digitizing with or without adding a vertex

I propose to add an option to determine right-click behevior in digitalization: it adds or not a vertex.
This adds a new option in the digitizing category in main option:

![aaa](https://f.cloud.github.com/assets/127259/854722/5c013928-f50d-11e2-90a6-cdeab82c2104.png)