Provide a way to customise keyboard shortcuts on CodeMirror

**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) https://github.com/Ionaru/easy-markdown-editor/issues/124#issuecomment-596619936. 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.

Start by inspecting how EasyMDE exposes and initializes its underlying CodeMirror instance, then review issue #124 and its linked comment. Verify whether easyMDE.codemirror can customize CodeMirror keymaps such as selectAll; done means either the customization works with documented usage or the feature and its documentation are added.

Written by the indexing model from the issue text.