Enable changing settings on a buffer-basis. QueryCommandComplete new
tries to read every configuration setting from the buffer-level
variables (b:qcc_*) before reading the global score ones (g:qcc_*).

This means that now you can have different buffers calling completely
different completion functions (and options).

This patch builds on top of the idea bootstrapped by @equalsraf
on his branch[1].

[1]: https://github.com/equalsraf/querycommandcomplete.vim/commit/7e9a7a602f4ae9c0355ca27cada4ba7173c6ec59