Add Github workflow file

[MarkKremer]

• Added Github Actions to build the packages, run gofmt, go test and upload the coverage to coveralls.
• Added a build status & code coverage badge in readme.
• I didn't add the libgl1-mesa-dev dependency listed in the README because it worked without it.

Do this before merging

1. Go to coveralls.io
2. Login or create an account. Easiest is to use Github as you'll probably need to connect to it anyway.
3. Enable the Pixel repo. You'll need sufficient access rights to do this.
4. Take note of the token.
5. Add the token as a secret to the Github Actions under under Security → Secrets and variables → Actions → Secrets → New repository secret. Give it the name COVERALLS_REPO_TOKEN.

6. Repeat for all other gopxl repo's. I'll make PR's for them too.