Merge fdbb9cfe26 into 640b324401

and also expecting `file` instead of `source` as env. variable to make that possible without dancing around: 
```
#!/bin/bash

file=backup && yadm bootstrap;
file=install && yadm bootstrap;
file=configure &&yadm bootstrap;
```

contrib/bootstrap/bootstrap-source

@@ -0,0 +1,94 @@
+#!/bin/bash
+
+# Save this file as ~/.config/yadm/bootstrap and make it executable. It expects
+# environment variable `file` with a name of shell script to execute.
+# `file` can be relative to ~/.config/yadm/ or has full path.
+#
+#  Usage:
+#    file=install yadm bootstrap
+#  or
+#    file=~/.config/yadm/install yadm bootstrap
+#
+# where `~/.config/yadm/install` can be like this:
+#
+# [[ ! $(type -t install) = 'function' ]] && echo "Usage: file=$(basename "$0") yadm bootstrap" && exit 1
+#
+# confirm yes 'softwareupdate --agree-to-license --install --all' "$(info 'Install ' warning 'OSX updates')"
+# install 'brew' '/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"'
+#
+# brew bundle --global
+#
+
+TEXT_COLOR_RED=$(tput setaf 1)
+TEXT_COLOR_GREEN=$(tput setaf 2)
+TEXT_COLOR_BLUE=$(tput setaf 4)
+TEXT_COLOR_YELLOW=$(tput setaf 3)
+TEXT_DEFAULT=$(tput sgr0)
+
+info() {
+	printf "${TEXT_COLOR_BLUE}%s${TEXT_DEFAULT}" "$1"; "${@:2}"
+}
+
+error() {
+	printf "${TEXT_COLOR_RED}%s${TEXT_DEFAULT}" "$1"; "${@:2}"
+}
+
+success() {
+	printf "${TEXT_COLOR_GREEN}%s${TEXT_DEFAULT}" "$1"; "${@:2}"
+}
+
+warning() {
+	printf "${TEXT_COLOR_YELLOW}%s${TEXT_DEFAULT}" "$1"; "${@:2}"
+}
+
+source_if_exists() {
+	local src=$1
+	local verbose="${2:-no}"
+
+	[[ ! -f $src ]] && return 1;
+	[[ "${verbose:0:1}" == v* ]] && printf "%s\n\n" "$(info "-> $src")";
+	source "$src"
+}
+
+command_exists () {
+	command -v "$1" &> /dev/null
+}
+
+# install a 'command' via 'code' if it was not previously installed yet
+install() {
+	local command=$1
+	local code=$2
+	local message=$(info 'Installing ' warning "$command")
+
+	if command_exists "$command"; then
+		echo "$message...[$(success 'already')]"
+	else
+		echo "$message..."
+		eval "$code"
+	fi
+}
+
+# execute a 'code' after confirmation from user
+confirm() {
+	local default_answer="${1:-no}"
+	local code=$2
+	local message=$3
+	local options answer
+
+	if [[ "${default_answer:0:1}" == y* ]]; then
+		options="$(success 'YES')/no"
+	else
+		options="yes/$(success 'NO')"
+	fi
+
+	read -r -p "${message} [${options}]:" answer
+	answer=${answer:-$default_answer}
+
+	if [[ "${answer:0:1}" == y* ]]; then
+		eval "$code"
+	fi
+}
+
+source_if_exists "$(dirname "${0}")/$file" v ||
+source_if_exists $file v ||
+([[ -z $file ]] && echo "Usage: file=FILENAME yadm bootstrap" || echo "Can't locate file '$file' for bootstrapping")