Add flash light support

[ribx]

I would like to see the flash light blinking when a notification arrives.

Possible options:

• number of flashes
• duration of a flash
• maximum brightness (think this needs root)

[lkorth]

Seems like a good option to have, however I haven't had any requests for it before so it's pretty far down the list. Pull requests are welcome if you would like to add it.

[dsvetlov]

+1 for that request