Extend API to fetch headlines from agenda

[seflue]

To allow refiling with telescope-orgmode.nvim from the agenda, I need an API function to fetch the headline of the current cursor position in the agenda. I would be motivated to propose a PR, but because I am unfamiliar with the agenda implementation, I actually don't know exactly where to start. @kristijanhusak I would appreciate if you can guide me to some entry points and have a discussion about how to implement such an API function, which fits your design philosophy in Nvim-Orgmode.

[seflue]

For reference here the corresponding issue #25 in telescope-orgmode.