Add Travis CI with luacheck

.luacheckrc

@@ -0,0 +1,16 @@
+-- Rerun tests only if their modification time changed.
+cache = true
+
+-- Glorious list of warnings: https://luacheck.readthedocs.io/en/stable/warnings.html
+ignore = {
+  "212", -- Unused argument, In the case of callback function, _arg_name is easier to understand than _, so this option is set to off.
+  "411", -- Redefining a local variable.
+  "412", -- Redefining an argument.
+  "422", -- Shadowing an argument
+  "122" -- Indirectly setting a readonly global
+}
+
+-- Global objects defined by the C code
+read_globals = {
+  "vim",
+}