660 comes from Xorg, but feels kind of slow. The wlroots and sway
default is 600, which is a little better.
The 25 repeat interval matches between Xorg (where it is defined in
terms of interval, rather than rate) and wlroots.
Add option to let the users configure the keyboard repeat delay and
rate. Also change the default values to mimic the Xorg, which is
also the values used by default by the majority of the compositors.
Fixes: #7
Ivan Molodetskikh