Add Travis CI with luacheck

.gitignore

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 doc/tags
+.luacheckcache

.luacheckrc

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+-- 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",
+}

.travis.yml

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+
+language: c
+dist: bionic
+before_install:
+  - sudo apt-get update
+  - sudo add-apt-repository universe
+  - sudo apt install luarocks -y
+  - sudo luarocks install luacheck
+
+script:
+  - luacheck **/**/*.lua --codes