force autocommand processing for the BufReadPost group

I'm not totally sure this is the right solution to the problem, but my change works as expected.

For some reason when setting vim.current.buffer in the python code, any modelines in the note are not processed by vim. Forcing a call to doautocmd triggers modeline processing. I picked the BufReadPost group as that made the most sense.

I like to use modelines in my notes to set the FileType. Specifically txt files formatted as votl/vimorganizer files.