Create a space from script on El Capitan
I do not have Slate window manager installed however the following AppleScript code will add a new Desktop in Mission Control in a normal install of OS X El Capitan and other versions of OS X and macOS.

do shell script "open -a 'Mission Control'"
delay 0.5
tell application "System Events" to click (every button whose value of attribute "AXDescription" is "add desktop") of group 1 of process "Dock"
delay 0.5
tell application "System Events" to key code 53