ENH: Add DistractFree#Active() accessor for active state.

I'd like to have a custom toggle function that performs more work around toggling. For that, the current state of the plugin needs to be exposed. Such an accessor is also helpful for other customizations that need to consider the plugin's state.
Merge pull request #2 from inkarkat/enh-accessor-for-active-state

ENH: Add DistractFree#Active() accessor for active state.