Hybrid line number mode

[andyau-crunchyroll]

Now that relative line number has been partially implemented (PR# 343), hybrid line number mode would make a nice addition.

Hybrid line number is a combination of relative and absolute line numbers, where all line numbers in the editor are relative to the current line, except for the currently selected line. Instead, the currently selected line will display it's absolute line number.

Expected Behavior:
Setting number and relativenumber in the user's .xvimrc config will enable hybrid line number behavior.

Working example in VS Code + Vim plugin

Actual Behavior:
Xcode is showing line numbers as relative only (as if only relativenumber is enabled).

Current behavior in Xcode + XVim2