Add CKEditor-TeXZilla plugin

Link to CKEditor-TeXZilla: https://github.com/r-gaia-cs/CKEditor-TeXZilla

For this commit it was used c4e484c7f9fd71f5f313892a3da2919ee2b14722
Update based on version 1.0
Merge pull request #2038 from r-gaia-cs/TeXZilla

Add basic math editing features