Add helpers for command completion

Vim's command completion interface is fine for simple commands, but pretty awkward for more complex commands like :Glaive that can't necessarily base completion just on `ArgLead` (the last partial arg on the command line), and have to do complex processing on `CmdLine` using `CursorPos` and parse out the command name, leading modifiers and whatnot to handle command lines like ": ::4,'XGlaive! …".

Maktaba should include some utilities to bridge the gap between vim's completion interface and what you need to write powerful completion functions easily.