Add ergo-mode-hook to Emacs ergo-mode and run it

[rpgoldman]

Is your feature request related to a problem? Please describe.
I expect to be able to customize the behavior of ergo-mode, and the standard way to do this for an Emacs major mode is to have a mode-hook.

Describe the solution you'd like
Add ergo-mode-hook variable (see Hooks chapter of Emacs lisp manual and Major Modes description) per this

Each major mode should have a normal mode hook named modename-mode-hook. The very last thing the major mode command should do is to call run-mode-hooks. This runs the normal hook change-major-mode-after-body-hook, the mode hook, the function hack-local-variables (when the buffer is visiting a file), and then the normal hook after-change-major-mode-hook.

(defvar ergo-mode-hook)

and revise the last line of ergo-mode to (run-mode-hooks 'ergo-mode-hook 'flora-mode-hook) currently ergo-mode only runs flora-mode-hook which isn't what Emacs users will expect.

If you prefer, I can provide a Merge Request for this.