Add `onUploading` for `imageUploadFunction`

[bilogic]

Is your feature request related to a problem? Please describe. When remote server is slow and I paste/upload an image, there is no reaction in easyMDE and make users wonder if they did it correctly, some may try to paste again leading to double uploads

Describe the solution you'd like Show a progress indicator like GitHub ![Uploading image.png…]() and replace it with the actual link thereafter. From the code, I think it make sense to add onUploading and onUploaded if necessary, i.e.

imageUploadFunction: function(file, onSuccess, onError, onUploading) {
    // ... some validation
    onUploading("Uploading image.png...")

Describe alternatives you've considered none

Additional context none