Update src/index.html:Added option for encoding

Added lines in index.html @id="template-open" & @id="template-save" where you can input the key for encoding and decoding all text in mindmap. 
If the fields are empty, then the encoding/decoding functions do not kick in.
Else, the functions in src/OpenDocument@view.openExernalFileClicked and src/Storage.js@saveDocument will do apply the key to the text.