Introduce a new configurable option that allows to customize text selection

Right clicking within CodeMirror editor, but outside of an existing selected text clears the selection.

This pull request introduces a new option that allows to customize the behavior.

Not sure about the option name. Any better suggestions?

See also this thread:
https://groups.google.com/forum/#!topic/codemirror/kzy1oGnmhBI

Honza