Right now, only the file name supports being either a string, symbol or
function, etc. Nonetheless, it would be nice if headline could be a
function that takes in as parameter a list of all headlines present in the
file so that we can dynamically pick a headline under which we want to add
an entry.  The function can also return a string to represent a
(potentially) new headline to insert.

My use case for that is I have an org file in which each headline is a
specific time, and I want to reuse a headline that is only within 5 minutes
behind the current time.