Atom is a free and open-source text and source code editor for OS X, Linux, and Windows with support for plug-ins written in Node.js, and embedded Git Control, developed by GitHub - wikipedia 
Atom is a desktop application built using web technologies. Most of the extending packages have free software licenses and are community-built and maintained.
Atom is based on Electron (formerly known as Atom Shell),[9] a framework that enables cross-platform desktop applications using Chromium and io.js, which is also developed by GitHub. It is written in CoffeeScript and Less. It can also be used as an IDE.
There is also command line --help:
atom --help Atom Editor v0.80.0 Usage: atom [options] [file ...] Options: -d, --dev Run in development mode. [boolean] -f, --foreground Keep the browser process in the foreground. [boolean] -h, --help Print this usage message. [boolean] -l, --log-file Log all output to file. [string] -n, --new-window Open a new window. [boolean] -s, --spec-directory Set the spec directory (default: Atom's spec directory). [string] -t, --test Run the specified specs and exit with error code on failures. [boolean] -v, --version Print the version. [boolean] -w, --wait Wait for window to be closed before returning. [boolean]