feat: add GPL license

[lefamoffat]

3. GPL

• This is the heaviest license.
• Choose this if you're afraid of people using your work in an unfair way (e.g. make money on your back bothers you). If they use your software, they will have to use the same license as you (i.e. probably open-source it).
• In a nutshell, it says: if you make a derivative work of this, and distribute it to others under certain circumstances, then you have to provide the source code under this license.
  It's a copyleft license (as opposed to copyright).
• For example, one of the reasons why Apple's Mac OS is not built on top of Linux is because Linux is GPL, which means that OSX would have been forced to be opened sourced to.
• A lot of startups don't like the GPL license because it can pose a serious problem during an exit. Indeed, it's rare the investors would like to buy something that they're forced to open-source.

https://gist.github.com/nicolasdao/a7adda51f2f185e8d2700e1573d8a633#3-gpl

[dankain]

I think we should also update the package.json to say GPL, at the moment it says unlicensed