add vim8 popup functionality to winopen()

[ferguscollins]

-basic framework for popup window to address #7
this introduces a bug with the following error message:

Error detected while processing function leaderGuide#start_by_prefix[20]..<SNR>37_start_buffer[2]..<SNR>37_create_string:
line    9:
E117: Unknown function:1
E702: Sort compare function failed
Error detected while processing function leaderGuide#start_by_prefix[20]..<SNR>37_start_buffer:
Line 8:

the original code is commented out for reference while debugging, obviously can be removed when pull request is functional

[ferguscollins]

Ah I didn't realise commits in forks also reference issues in the main repository. Nonetheless I am stuck at this error and it is late where I am so if you know why this is happening then please let me know.

Edit: I realise that the first error refers to here and the second error refers to here but do not know an alternative for nvim_buf_set_lines or why the sort comaprison fails.