Publicly expose _toggleLink/_replaceSelection

It's currently impossible to perform text-selection modifications from a custom button using the same logic as EasyMDE.

The only workaround right now is outright copy/pasting these functions to your own code.

Start by locating the _toggleLink and _replaceSelection functions in the EasyMDE source and review how the editor invokes them for text selection changes. Make the same logic publicly accessible for custom buttons without requiring users to copy the functions, then verify the existing editor behavior remains unchanged.

Written by the indexing model from the issue text.