feat: add status_icons configuration option

[antinomie8]

Lets the user configure the git status icons displayed in the file tree.
I doubt the get_status_icon function has anything to do in hl.lua, perhaps it should be moved into another file.
It lets the user define custom icons for git status letters as such, for instance (nerd fonts):

status_icons = {
	["M"] = "", -- modified
	["T"] = "T", -- type
	["A"] = "", -- added
	["D"] = "󱟃", -- deleted
	["R"] = "", -- renamed
	["C"] = "", -- copied
	["U"] = "", -- updated but unmerged
	["X"] = "X", --
	["B"] = "B", --
	["?"] = "", -- untracked
	["!"] = "", -- ignored
}

[antinomie8]

closes #532