lua: make sure lpeg is in fact optional

fixes #1119: lua: lpeg module isn't actually optional

lua/plugins/filetype.lua

@@ -500,6 +500,7 @@ vis.events.subscribe(vis.events.WIN_OPEN, function(win)
 		for _, cmd in pairs(filetype.cmd or {}) do
 			vis:command(cmd)
 		end
+		if not vis.lexers.property then return end
 		local path = vis.lexers.property['lexer.lpeg.home']:gsub(';', '/?.lua;') .. '/?.lua'
 		local lexpath = package.searchpath('lexers/'..syntax, path)
 		if lexpath ~= nil then

lua/vis-std.lua

@@ -17,6 +17,7 @@ vis:option_register("theme", "string", function(name)
 	local lexers = vis.lexers
 	lexers.lexers = {}
 
+	if not lexers.load then return false end
 	if not lexers.property then lexers.load("text") end
 	local colors = lexers.colors
 	local default_colors = { "black", "red", "green", "yellow", "blue", "magenta", "cyan", "white" }