Re-implement CLI in Go, automate release process

[djones6]

This PR replaces the previous Node-based implementation of kitura-cli with a statically compiled Go binary.

I've (mostly) automated the release process as part of this:

• for normal commits to eligible branches (master, issue.*), Travis will run a test build with a default version (0.0.1).
• for tags that follow the SemVer (x.y.z) format, Travis will build the CLI with that version number, generate downloadable packages and attach the outputs to the release:
  • kitura-cli_<release>_amd64.deb: A Debian package suitable for installing on Ubuntu with dpkg
  • kitura-cli_<release>_darwin.tar.gz: A package suitable for installing via Homebrew
  • install.sh: A convenience script that enables a one-liner install (see README.md)
  • kitura.rb: An updated formula for the homebrew-kitura repo (see Release-Process.md)

Once a release has been tagged and the build has completed, the only manual step required is that the kitura.rb file is updated via a PR to the homebrew-kitura repo. This could be automated in the future.

Resolves #53

[djones6]

@ianpartridge Temporary test release tagged from this branch: https://github.com/IBM-Swift/kitura-cli/releases/tag/0.0.17

Brew formula can be tested via my fork: https://github.com/djones6/homebrew-kitura

brew uninstall kitura
brew untap ibm-swift/kitura
brew tap djones6/kitura
brew install kitura
kitura --version

Linux install can be tested via docker:

docker run -it ubuntu:18.04 /bin/bash
apt-get update && apt-get install -y curl sudo
curl -fsSL https://github.com/IBM-Swift/kitura-cli/releases/latest/download/install.sh | sudo bash
kitura --version

[djones6]

Both platforms can now be installed via the one-liner install.sh route, and README has been updated to include manual install steps for Mac too.
Tested via release 0.0.18: https://travis-ci.org/IBM-Swift/kitura-cli/builds/573760648