I bet these things have annoyed you and the relief I feel getting a solution is worth sharing...
No quicklook on markdown
brew cask install qlmarkdown
No quicklook on json files
brew cask install quicklook-json
Not being able to see invisible (dot) files
Either run this command
defaults write com.apple.finder AppleShowAllFiles YES, or, for a fuller experience, download Tikertool - https://www.bresink.com/osx/0TinkerTool/download.php