ci: replace Travis CI with Github actions for continuous build and test runs

[mewmew]

Anyone who feels like contributing, we'd like to update our CI pipeline (from Travis CI) to use GitHub actions to build and test mewkiz/flac.

The test action should include git submodule update --init --recursive to download the IETF test cases (i.e. the mewkiz/flac/testdata/flac-test-files/ Git submodule) prior to running go test github.com/mewkiz/flac/... (see #60 (comment) for an example).

If it's possible to cache the download of IETF test cases in such a way that they don't have to be re-downloaded each time, that would also be wonderful since the test cases amount to some 250 MB.

Cheers,
Robin