[Feature Request] jump and help

[seagle0128]

Hi, I love this amazing package and I'm enjoying!

I have two requirements. Can you think about them?

1. Jump to the first and the last. I have the implementation for it.

   
   (defun symbol-overlay-switch-first ()
     (interactive)
     (let* ((symbol (symbol-overlay-get-symbol))
            (keyword (symbol-overlay-assoc symbol))
            (a-symbol (car keyword))
            (before (symbol-overlay-get-list a-symbol 'car))
            (count (length before)))
       (symbol-overlay-jump-call 'symbol-overlay-basic-jump (- count))))

(defun symbol-overlay-switch-last () (interactive) (let* ((symbol (symbol-overlay-get-symbol)) (keyword (symbol-overlay-assoc symbol)) (a-symbol (car keyword)) (after (symbol-overlay-get-list a-symbol 'cdr)) (count (length after))) (symbol-overlay-jump-call 'symbol-overlay-basic-jump (- count 1))))

I bind to `<` and `>` in ` symbol-overlay-map`.

I can make a PR if you agree.

2. Show help for keybindings. Suggest to bind to `h`.

BTW: `doom-modeline` supports `symbol-overlay`. Refer to https://emacs-china.org/t/package-symbol-overlay-symbol/7706/18?u=seagle0128. 

[purcell]

For the jumping, see #28.

I've added a help binding in 39a2ad8.

Thanks!