Disable EasyMDE inside form

### I'm submitting a...
- [] Bug report
- [x] Feature request

### Version information
Browser type and version: latest Chrome
EasyMDE version: 2.5.1

Hi, thank you for the awesome plugin and big thanks for the regular updates.

I'm here to ask you about "disabling" feature. I've used your plugin in Formik (Reactjs) and in some cases, I should disable all fields in the form and EasyMDE too.

My feature request is: Can we have "disable" option or not.

Thank you.