allow specifying an elisp function to provide folded-state-text based on first line of fold

[edgimar]

It would be nice to be able to specify a regular expression (or more generally an elisp function) that would be applied to the first folded line in order to extract some portion of that line for showing as the origami-fold-replacement text for a particular fold.

In other words, if I have:

# {{{
def foobar(a, b, c):
    pass
# }}}

then after folding, I should be able to see:

# {{{ foobar(a, b, c) ... }}}

provided that I have set up an appropriate elisp function to convert the first line into the replacement text.

Or it might be even better to be able to replace the entire folded-state line, and not just the stuff between {{{ and }}}, so you could have something like the following if you want:

# ---- def foobar(a, b, c) ... ----