Provide a way to customise keyboard shortcuts on CodeMirror

[tadayosi]

Is your feature request related to a problem? Please describe.
EasyMDE provides a way to customise the default keyboard shortcuts here:
https://github.com/Ionaru/easy-markdown-editor#keyboard-shortcuts
However, the defined actions don't cover what are defined in underlying CodeMirror, thus for instance shortcuts such as Ctrl+A (selectAll) cannot be customised.

Describe the solution you'd like
Provide a way to also customise the keyboard shortcuts https://codemirror.net/5/doc/manual.html#keymaps on CodeMirror from EasyMDE.

Describe alternatives you've considered
If it's already possible in some way (e.g. #124), then add explanation to the documentation on how to do so.

Additional context
This might be related or even a duplicate of #124, and someone suggested a solution (not sure if it works) #124 (comment). So it could be that easyMDE.codemirror is the way to go, but I'd like to make sure that it really works if it is.