Fix shortkeys for windows and non-US/UK keyboards about mercury

[tmhglnd]

Added a Setup Shortkeys menu under Settings in 03994c4. The keybindings are stored under ~/Documents/Mercury/Preferences/shortkeys.json.

The default keys are currently as follows for Mac:

"execute" : [ "alt-return", 2044 ],
"silence" : [ "alt-.", 2094 ],
"comment" : [ "alt-/", 2095 ],
"disable-editor" : [ "alt-,", 2092 ],
"delete-line" : [ "alt-x", 2168 ],
"copy-line" : [ "alt-c", 2147 ],
"paste-line" : [ "alt-v", 2166 ],
"paste-replace-line" : [ "alt-p", 2160 ],
"jump-top" : [ "alt-up", 2039 ],
"jump-bottom" : [ "alt-down", 2038 ],
"jump-begin" : [ "alt-left", 2037 ],
"jump-end" : [ "alt-right", 2036 ],
"up" : [ "alt-w", 2167 ],
"down" : [ "alt-s", 2163 ],
"left" : [ "alt-a", 2145 ],
"right" : [ "alt-d", 2148 ]

[tmhglnd]

Also see https://github.com/tmhglnd/mercury/blob/master/docs/06-shortkeys.md for more information