Add Bash completion script (#60)

* Add completion script
* Add introspection

completion/README.md

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+# Prerequisites
+
+**yadm** completion only works if Git completions are also enabled.
+
+# Installation
+
+## Homebrew
+
+If using `homebrew` to install **yadm**, completions should automatically be handled if you also install `brew install bash-completion`. This might require you to include the main completion script in your own bashrc file like this:
+
+```
+[ -f /usr/local/etc/bash_completion ] && source /usr/local/etc/bash_completion
+```
+
+## Manual installation
+Copy the completion script locally, and add this to you bashrc:
+```
+[ -f /full/path/to/yadm.bash_completion ] && source /full/path/to/yadm.bash_completion
+```

completion/yadm.bash_completion

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+# test if git completion is missing, but loader exists, attempt to load
+if ! declare -F _git > /dev/null && declare -F _completion_loader > /dev/null; then
+  _completion_loader git
+fi
+
+# only operate if git completion is present
+if declare -F _git > /dev/null; then
+
+  _yadm() {
+
+    local current=${COMP_WORDS[COMP_CWORD]}
+    local penultimate=${COMP_WORDS[COMP_CWORD-1]}
+    local antepenultimate=${COMP_WORDS[COMP_CWORD-2]}
+
+    local GIT_DIR
+    # shellcheck disable=SC2034
+    GIT_DIR="$(yadm introspect repo 2>/dev/null)"
+
+    case "$penultimate" in
+      bootstrap)
+        COMPREPLY=()
+        return 0
+      ;;
+      config)
+        COMPREPLY=( $(compgen -W "$(yadm introspect configs 2>/dev/null)") )
+        return 0
+		  ;;
+      decrypt)
+        COMPREPLY=( $(compgen -W "-l" -- "$current") )
+        return 0
+		  ;;
+      init)
+        COMPREPLY=( $(compgen -W "-f -w" -- "$current") )
+        return 0
+		  ;;
+      introspect)
+        COMPREPLY=( $(compgen -W "commands configs repo switches" -- "$current") )
+        return 0
+		  ;;
+      help)
+        COMPREPLY=() # no specific help yet
+        return 0
+      ;;
+      list)
+        COMPREPLY=( $(compgen -W "-a" -- "$current") )
+        return 0
+		  ;;
+    esac
+
+    case "$antepenultimate" in
+      clone)
+        COMPREPLY=( $(compgen -W "-f -w --bootstrap --no-bootstrap" -- "$current") )
+        return 0
+		  ;;
+    esac
+
+    # this condition is so files are completed properly for --yadm-xxx options
+    if [[ ! "$penultimate" =~ ^- ]]; then
+      # TODO: somehow solve the problem with [--yadm-xxx option] being
+      #       incompatible with what git expects, namely [--arg=option]
+      _git
+    fi
+    if [[ "$current" =~ ^- ]]; then
+      local matching
+      matching=$(compgen -W "$(yadm introspect switches 2>/dev/null)" -- "$current")
+      __gitcompappend "$matching"
+    fi
+
+    if [ "$COMP_CWORD" == 1 ] || [[ "$antepenultimate" =~ ^- ]] ; then
+      local matching
+      matching=$(compgen -W "$(yadm introspect commands 2>/dev/null)" -- "$current")
+      __gitcompappend "$matching"
+    fi
+
+    # remove duplicates found in COMPREPLY (a native bash way could be better)
+    if [ -n "${COMPREPLY[*]}" ]; then
+      COMPREPLY=($(echo "${COMPREPLY[@]}" | sort -u))
+    fi
+
+  }
+
+	complete -o bashdefault -o default -F _yadm yadm 2>/dev/null \
+		|| complete -o default -F _yadm yadm
+
+fi