Update control code to change cursor shape, 50 to 1337

[bootleq]

Hi, the control code to change cursor shape (when starting/ending input in Vim) is a bit old.
It is OSC 50 (at d24ab42#L105-L112) which has been deprecated about 2015 by iTerm2 533930d8, because conflict to xterm.

Could we update it to 1337 as iTerm2 does?
Though most user just use 50 fine, please consider the case, if we do some co-work sharing Vim session with others, they might encounter trouble their terminal got unexpected code.

For example when I use mintty and connect to maximum-awesome user's tmux session, every time switching mode in Vim produce below prompt (with OSC 1337 the problem is gone):

[rudle]

Thanks for reporting the issue. Would you mind taking the time to prepare a pull request so that we may test out this change?