Add a new function fish_mode_prompt which (if it is defined) has its output
prepended to the left prompt. Rather than replacing the prompt wholesale, make
fish_vi_mode enable this function by setting a variable __fish_vi_mode. This
enables vi mode to interoperate nicely with custom prompts. Users who want
to change how the mode is reported can either redefine this function or
erase it entirely. Fixes#1988.
