Truncate path if too long

[KackaHannah]

I added a function that will shorten the path to the current directory if it is longer than 8 characters. All directory names in the path except the current directory are replaced by their first letter.

This functionality should behave very similarly to the one implemented in fish.

Preview:

 ~ » cd ts
 ~/ts » cd shr
 ~/ts/shr » cd even\ longer\ directory\ name\ with\ spaces
 ~/t/l/even longer directory name with spaces » cd last_dir
 ~/t/l/e/last_dir »