Allow user option to keep `data-pg-collapsed` in saved html files

In order for PG to keep track of folded parts of the html tree, the data-pg-collapsed string is added to the element where the folding takes place… this text is then removed upon reading the file into PG.

this effectively means only PG can edit the html in real time.

if using an external editor like atom or VSC to work in both simultaneously, this string will get lost in the sync communication between PG and the external editor.

i suggest PG allow the user to decide if they want to keep the string permanently in their html code rather than allow PG to strip it off when it reads in the file. That way those who want to use an external editor will still get to enjoy the benefits of a folded tree structure.

I have found the current model very annoying.

Thanks for the suggestion @droidgoo

+1 for this one!