Add gosetup for easily installing Go

#!/usr/bin/env bash

#

# Copyright 2019  Dominik Sekotill <dom.sekotill@kodo.org.uk>

# 

# Licensed under the Apache License, Version 2.0 (the "License");

# you may not use this file except in compliance with the License.

# You may obtain a copy of the License at

# 

#     http://www.apache.org/licenses/LICENSE-2.0

# 

# Unless required by applicable law or agreed to in writing, software

# distributed under the License is distributed on an "AS IS" BASIS,

# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

# See the License for the specific language governing permissions and

# limitations under the License.

set -eu -o pipefail

shopt -s extglob

case $0 in

	?(*/)go1.*)

		versions=( ~/.local/$(basename "$0")* )

		export GOROOT=${versions[-1]}

		exec ${GOROOT}/bin/go "$@"

		;;

esac

# Install Go-$VERSION

case `uname -m` in

	x86_64|amd64) ARCH=amd64 ;;

	i686*|i386) ARCH=386 ;;

	armv6l|armv7l) ARCH=armv6l ;;

	arm64|aarch64) ARCH=arm64 ;;

esac

KERNEL=$(uname -s | tr '[[:upper:]]' '[[:lower:]]')

PATTERN="https:\\/\\/dl.google.com\\/go\\/go${VERSION-}[0-9.]*\\.${KERNEL}-${ARCH}\\.tar\\.gz" 

URL=$(wget -qO- https://golang.org/dl/ | grep -o "${PATTERN}" | head -n1) || true

if [[ ${#URL} -eq 0 ]]; then

	echo >&2 "Could not find a link for version: ${VERSION-latest}"

	echo >&2 "(tried ${PATTERN})"

	exit 1

fi

if [[ -z "${VERSION-}" ]]; then

	SET_DEFAULT=y

fi

VERSION=$(basename $URL)

VERSION=${VERSION%.${KERNEL}-${ARCH}.tar.gz}

VERSION=${VERSION#go}

PRIME_VERSION=${VERSION%.${VERSION#*.*.}}

if [[ -x ~/.local/go${VERSION}/bin/go ]]; then

	echo >&2 "Go ${VERSION} appears to already be installed"

elif ! mkdir ~/.local/go${VERSION}; then

	echo >&2 "Please remove this directory/file to continue installation"

	exit 1

else

	wget -O- $URL | tar -xz -C ~/.local/go${VERSION} --strip-components 1

fi

ln -srf $0 ~/.local/bin/go${VERSION}

ln -srf $0 ~/.local/bin/go${PRIME_VERSION}

if [[ -n "${SET_DEFAULT-}" ]]; then

	rm -f ~/.local/go

	ln -sr ~/.local/go${VERSION} ~/.local/go

fi