Add a config option to set the EDITOR kitty uses

Useful on systems that make it hard to set system-wide environment
variables. See #580

kitty/constants.py

@@ -4,7 +4,6 @@
 
 import os
 import pwd
-import shlex
 import sys
 from collections import namedtuple
 
@@ -16,7 +15,6 @@ str_version = '.'.join(map(str, version))
 _plat = sys.platform.lower()
 is_macos = 'darwin' in _plat
 base = os.path.dirname(os.path.abspath(__file__))
-editor = shlex.split(os.environ.get('EDITOR', 'vim'))
 
 
 ScreenGeometry = namedtuple('ScreenGeometry', 'xstart ystart xnum ynum dx dy')