Migrate to GitHub Actions

[asahasrabuddhe]

Aim

This PR closes #1465.

Changes

Added .github/workflows/echo.yml file to enable GitHub Actions. The current implementation has the following steps running on Ubuntu, macOS, and Windows:

1. Run the tests on the current three supported versions of Go: 1.11, 1.12, and 1.13.
2. Upload results to codecov.io. This feature is currently incomplete as I do not have this repository's codecov token. I anticipate a discussion where we would discuss just putting the token in plain text for the world to see but this is what we need to do. If, for example, we create a secret and access that secret within the workflow, this secret would ONLY be accessible for builds running in this repository. If a user has forked this repository, this token won't be passed down to them. This would make it impossible for those users to check for coverage before making a PR. If we are comfortable with this behavior, we can use the secret too. If not, either send me the token or feel free to make a commit and paste the token on line 52.

[OPTIONAL] Creating a release. GitHub Actions also support creating automated releases. If this is something we are interested in, we can work on a method to generate the release notes and have the automated release step implemented. How it would work is that every time a tag is pushed, the release step would be executed creating a new release.

For a sample output of the workflow, please head over to: https://github.com/asahasrabuddhe/echo/actions

[codecov]

Codecov Report

Merging #1473 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1473   +/-   ##
=======================================
  Coverage   84.32%   84.32%           
=======================================
  Files          27       27           
  Lines        2073     2073           
=======================================
  Hits         1748     1748           
  Misses        212      212           
  Partials      113      113

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