superscript and subscript buttons

this PR replaces https://github.com/HackerWins/summernote/pull/254, https://github.com/HackerWins/summernote/pull/255, https://github.com/HackerWins/summernote/pull/256, https://github.com/HackerWins/summernote/pull/257 and https://github.com/HackerWins/summernote/pull/258

I didn't include the distribution files because I thought it might make it more difficult to merge this PR. I can easily add them if it helps!