Feature-Request: Add option to lower/raise brightness at a certain percentage

[pyriand3r]

Great tool, worked immediatly on my Lenovo Thinkpad. However there is one thing missing for me to make it super usefull:

ATM it is only possible to set a defined brightness in percentage. It would be great to be able to lower/raise the current brightness at a certain percentage.

What i have in mind is following:

1. The current brightness it at 25%
2. One issue the command set-brightness +5
3. The current brightness is raised by 5% to 30%

That would make it possible to use the script in a system-wide shortcut.