skinlist: Allow textures containing '-' characters

Fixes issue #110
Thanks to Bastrabun for the code suggestion

skinlist.lua

@@ -6,7 +6,7 @@ local dbgprint = false and print or function() end
 function skins.register_skin(path, filename)
 	-- See "textures/readme.txt" for allowed formats
 
-	local prefix, sep, identifier, extension = filename:match("^(%a+)([_.])([%w_.]+)%.(%a+)$")
+	local prefix, sep, identifier, extension = filename:match("^(%a+)([_.])([%w_.-]+)%.(%a+)$")
 	--[[
 		prefix:     "character" or "player"
 		sep:        "." (new) or "_" (legacy)

textures/readme.txt

@@ -6,6 +6,7 @@ List of accepted texture names
 
 Public skin available for all users:
 	character.[number or name].png
+	^ The allowed characters in "[number or name]" are "[A-z0-9_.-]+".
 
 One or multiple private skins for player "[nick]":
 	player.[nick].png

unittest.lua

@@ -9,12 +9,14 @@ local function run_unittest()
 
 	-- -----
 	-- `.`: Simple register + retrieve operations
-	skins.register_skin(PATH, "player.DotSep.png")
-	skins.register_skin(PATH, "player._DotSep_666_.1.png")
+	assert(skins.register_skin(PATH, "player.DotSep.png"))
+	assert(skins.register_skin(PATH, "player._DotSep_666_.1.png"))
+	assert(skins.register_skin(PATH, "character._DotSep_With-Dash-.png"))
 
 	assert(get_skin("player.DotSep"))
 	assert(get_skin("player._DotSep_666_.1"))
 	assert(get_skin("player.DotSep.1") == nil)
+	assert(get_skin("character._DotSep_With-Dash-"))
 
 	-- -----
 	-- Ambiguous skin names (filenames without extension). Register + retrieve
@@ -42,7 +44,6 @@ local function run_unittest()
 	assert(get_skin("player_Com_Pat_42") == "player._Com_Pat_42")
 	assert(get_skin("player_Com_Pat_42_1") == "player._Com_Pat_42.1")
 
-
 	error("Unittest passed! Please disable them now.")
 end