Add snap package

[Taylor-C-Reese]

Is your feature request related to a problem? Please describe.
I'd like to easily and securely install gum on Ubuntu

Describe the solution you'd like
Add a snap package - https://snapcraft.io/

Describe alternatives you've considered
Installing from github is simple, but less reliable, predictable, and secure. That's why we have package managers. snap was made for situations like this.

Additional context
gum seems like a great project. I believe more people would use it if they could just do sudo snap install gum. Or, since the name gum is already taken, how about sudo snap install gumtool.