Add ShiftEnter plugin once its stabilised

[Reinmar]

ckeditor/ckeditor5-enter#54 is on review and should be merged soon.

However, we need to solve the following issues

• Cannot type after inserting break line element in a paragraph that ends with a space #1024
• https://github.com/ckeditor/ckeditor5-engine/issues/1422

before we'll be able to enable this plugin for everyone by default.

[Reinmar]

We should also remember about #766.

I realised that the current implementation of plainTextToHtml() doesn't create <br>s at all which meant that plain text was still pasted in an ugly way, despite the shiftenter feature being enabled.