Enable code linting feature

[sergeyklay]

This patch is still a work in progress and not finished yet. I will finish it soon, but I'm interested in @enderneko opinion on whether I should continue or not.

Usage:

Install luarocks:

# Ubuntu Linux
sudo apt-get install luarocks

# macOS
brew install luarocks

# Windows
choco install luarocks -y

Install luacheck:

luarocks install --local luacheck

Run check:

~/.luarocks/bin/luacheck . --codes

Progress:

• [x] Provide config for luacheck
• [ ] Provide linting at CI stage
• [ ] Fix code style (not finished)