improve

Previously, the players would have their selected skin reset after renaming
the skin textures to the dot separator. This commit implements skin name
migration to ease the transition for server owners.

See 'skins.__fuzzy_match_skin_name' for a detailed explanation.

API.md

@@ -27,6 +27,31 @@ Get all allowed skins for player. All public and all player's private skins. If
 Get all skins with metadata key is set to value. Example:
 skins.get_skinlist_with_meta("playername", playername) - Get all private skins (w.o. public) for playername
 
+## skins.register_skin(path, filename)
+Registers a new skin based on the texture file path specified by `path` and `filename`.
+
+ * `path` (string): points to the parent directory of the texture `filename`.
+   Generally, this should be in the format `mymod.modpath .. "/textures"`.
+ * `filename` (string): full file name, without any path specifications.
+   The file name must adhere to [one of the accepted naming formats](textures/readme.txt).
+
+Note: this function takes the following files into consideration:
+
+1. `<path>/<filename>` (required)
+    * Main skin texture
+2. `<path>/<filenamestem><separator>preview.png` (optional)
+    * Pre-generated preview image
+3. `<path>/../meta/<filenamestem>.txt` (optional)
+    * Metadata regarding the skin
+
+Return values:
+
+ * On failure: `false, reason`
+    * `reason` (string): human readable reason string (similar to `io.open` errors)
+ * On success: `true, key`
+    * `key`: unique skins key for use with e.g. `skins.get(key)` for subsequent
+      fine-tuning of the skin registration.
+
 
 ## skins.new(key, object)
 Create and register a new skin object for given key