Add GitHub workflow and LuaCheck (#34)

* Add GitHub workflow and LuaCheck

* Fix LuaCheck warnings

* Improve README.md

.github/workflows/build.yml

@@ -0,0 +1,11 @@
+on: [push, pull_request]
+name: build
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: lint
+        uses: Roang-zero1/factorio-mod-luacheck@master
+        with:
+          luacheckrc_url: https://raw.githubusercontent.com/minetest-mods/item_drop/master/.luacheckrc

.luacheckrc

@@ -0,0 +1,19 @@
+unused_args = false
+allow_defined_top = true
+max_line_length = 999
+
+ignore = {
+    "name", "drops", "i",
+}
+
+globals = {
+    "minetest",
+}
+
+read_globals = {
+    string = {fields = {"split", "trim"}},
+    table = {fields = {"copy", "getn"}},
+
+    "vector", "ItemStack",
+    "dump",
+}

README.md

@@ -1,15 +1,14 @@
-# item_drop
+# Item Drop [![](https://github.com/minetest-mods/item_drop/workflows/build/badge.svg)](https://github.com/minetest-mods/item_drop/actions) [![License](https://img.shields.io/badge/license-LGPLv2.1%2B-blue.svg)](https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html)
+
+A highly configurable mod providing item magnet and in-world node drops\
 By [PilzAdam](https://github.com/PilzAdam),
 [texmex](https://github.com/tacotexmex/), [hybriddog](https://github.com/hybriddog/).
 
-## Description
-A highly configurable mod providing item magnet and in-world node drops
-
 ## Licensing
 LGPLv2.1/CC BY-SA 3.0. Particle code from WCILA mod by Aurailus, originally licensed MIT.
 
 ## Notes
-item_drop can be played with Minetest 0.4.16 or above. It was originally
+`item_drop` can be played with Minetest 0.4.16 or above. It was originally
 developed by [PilzAdam](https://github.com/PilzAdam/item_drop).
 
 ## List of features

init.lua

@@ -103,7 +103,7 @@ if legacy_setting_getbool("item_drop.enable_item_pickup",
 		if pickup_particle then
 			local item = minetest.registered_nodes[
 				ent.itemstring:gsub("(.*)%s.*$", "%1")]
-			local image = ""
+			local image
 			if item and item.tiles and item.tiles[1] then
 				if inventorycube_drawtypes[item.drawtype] then
 					local tiles = item.tiles