add support for windows system-wide xonshrc

pypa/pip had a similar conversation about this a few years ago
(see: pypa/pip#309) and determined that
placing global configuration files in `%ALLUSERSPROFILE%\Application
Data` was the best solution without messing around with the registry.
Following their lead on this, specifically from
pypa/pip@2b5029c5

xonsh/environ.py

@@ -142,7 +142,9 @@ def xonshconfig(env):
     'XDG_CONFIG_HOME': os.path.expanduser(os.path.join('~', '.config')),
     'XDG_DATA_HOME': os.path.expanduser(os.path.join('~', '.local', 'share')),
     'XONSHCONFIG': xonshconfig,
-    'XONSHRC': (os.path.expanduser('~/.xonshrc'),'/etc/xonshrc') if ON_LINUX \
+    'XONSHRC': (os.path.expanduser('~/.xonshrc'),\
+                os.path.join(os.environ['ALLUSERSPROFILE'], 'Application Data', 'xonsh', 'xonshrc'))\
+               if ON_WINDOWS\
                else (os.path.expanduser('~/.xonshrc'),'/etc/xonshrc'), 
     'XONSH_CONFIG_DIR': xonsh_config_dir,
     'XONSH_DATA_DIR': xonsh_data_dir,