Add vim fold marker snippet

[Xvezda]

Using UltiSnips's ${VISUAL}.

Grab previous closest opening fold marker e.g.) {{{N, Check next closest closing fold marker N}}}.
Then parse N to compare both open/closing markers.

If two numbers are different, then cursor is out of folding context. So, create folding marker with number 1; {{{1, 1}}}.
Else, cursor is between of N fold markers. So, increase N+1 to create {{{N+1, N+1}}} fold markers.