Added a new feature and made some other changes. please review.

I've added a 'global_border' variable to all layouts that can be set in the theme which shrinks the windows and sets them toward the center of the monitor without effecting the useless gap; this can be modified dynamically in the same way as the useless gap. I've also modified the way the useless gap plays out as to keep the windows more even in their size in relation to each other, and with respect to the global border. lastly I've offset the window border to not escape the screen on the right hand and bottom sides. You may want to omit my changes to alsa.lua, this all started as my personal tuning, and then it caught on and was suggested to me that I should submit a pull request.