Add ins/ovr mode settings as input method

I have added this overwrite mode to the plugin to fix an issue when edition dates.
The default behaviour of the plugin is to insert typed character, as a consequence the end of the input might be lost, if the user didn't delete the content before inserting the new content.

Using a field with a date-time format (99/99/99 99:99) and try to change the month, you often end with this:

Before: ![before](https://f.cloud.github.com/assets/683264/729514/7ccfdc80-e235-11e2-82a3-13b488503c53.png)
While editing (change 01 to 02) : ![inserted](https://f.cloud.github.com/assets/683264/729515/82e34f76-e235-11e2-918a-82f16a80ac10.png)
After delete : ![deleted](https://f.cloud.github.com/assets/683264/729518/874ca814-e235-11e2-962b-2d5780fd157f.png)