Toolbar button for mentions and references

EasyMDE could provide toolbar buttons for mentions and references.
These buttons would only provide icons, tooltips and insert the character `@` and `#` into the editor. Nothing more.

It would be up to the user to implement hooks such as:
```js
easyMDE.codemirror.on('change', function(cm, change) {
if (change.text === '@') {
alert('Your mention picker here');
}
});
```
```js
easyMDE.codemirror.on('change', function(cm, change) {
if (change.text === '#') {
alert('Your references picker here');
}
});
```

No file or test is named. Start by locating EasyMDE's toolbar configuration and existing toolbar button definitions, then inspect how the editor inserts text and exposes icons and tooltips. Done means separate mention and reference buttons insert only `@` and `#`, with the requested icon and tooltip behavior.

Written by the indexing model from the issue text.