Suggested feature: Mouse tracking

When editing a long command in the command line it would be very convenient and natural to be able to click on a character and have the edit cursor moved to this character. I have seen this already working on Mac with Option+Click, but it doesn’t seem to work on my Linux machine. I am using Ubuntu and Gnome-Terminal, fish version is 2.7.1 and the $TERM is xterm-256color. It doesn’t work without customizations either.

I assume for this to work the shell should support [mouse tracking](https://stackoverflow.com/a/5970472/267780) but I am new to this and not 100% sure. Kind people in the gitter channel told me about Alt+V to start the default editor, and this is a definitely useful feature, but it seems an overkill for that particular task.

Steps to reproduce:
1) Start gnome-terminal with fish shell
2) Type a few characters in the prompt
3) Click one of these characters with the mouse

Actual: the editing cursor stays in its position
Expected: the editing cursor moves to the character clicked

Would you consider support for this feature?