Make it possible to change the mode-map prefix

The reason for this change is that the `C-c #` is often used and may therefore overwrite keybindings that already exist. Though it is possible to change every keybinding, we can now instead just change the prefix and keep the old bindings.

I would also suggest that the prefix is changed to something else, like `C-c C-m` since the emacs manual states that `C-c` followed by a single character should not be used for extensions, only for individuals' use.

> This set of keys, `C-c` followed by a single character, is strictly reserved for individuals' own use. (I call these 
> *own* keys, since these are for my own use.) You should always be able to create such a keybinding for your own
> use without stomping on someone else's keybinding. If you ever write an extension to Emacs, please avoid taking 
> any of these keys for public use. Create a key like `C-c C-w` instead. Otherwise, we will run out of *own* keys.
-- [emacs manual](https://www.gnu.org/software/emacs/manual/html_node/eintr/Keybindings.html)