Add an ability to install a specific version of pyenv (#126)

Author: cavcrosby

README.rst

@@ -19,6 +19,8 @@ Once prerequisites have been installed correctly:
 
 Install:
 ~~~~
+If you wish to install a specific release of Pyenv rather than the latest head, set the ``PYENV_GIT_TAG`` environment variable (e.g. ``export PYENV_GIT_TAG=v2.2.5``).
+
 .. code:: bash
 
     curl https://pyenv.run | bash

bin/pyenv-installer

@@ -29,7 +29,7 @@ failed_checkout() {
 }
 
 checkout() {
-  [ -d "$2" ] || git clone --depth 1 "$1" "$2" || failed_checkout "$1"
+  [ -d "$2" ] || git -c advice.detachedHead=0 clone --branch "$3" --depth 1 "$1" "$2" || failed_checkout "$1"
 }
 
 if ! command -v git 1>/dev/null 2>&1; then
@@ -43,12 +43,12 @@ else
   GITHUB="https://github.com"
 fi
 
-checkout "${GITHUB}/pyenv/pyenv.git"            "${PYENV_ROOT}"
-checkout "${GITHUB}/pyenv/pyenv-doctor.git"     "${PYENV_ROOT}/plugins/pyenv-doctor"
-checkout "${GITHUB}/pyenv/pyenv-installer.git"  "${PYENV_ROOT}/plugins/pyenv-installer"
-checkout "${GITHUB}/pyenv/pyenv-update.git"     "${PYENV_ROOT}/plugins/pyenv-update"
-checkout "${GITHUB}/pyenv/pyenv-virtualenv.git" "${PYENV_ROOT}/plugins/pyenv-virtualenv"
-checkout "${GITHUB}/pyenv/pyenv-which-ext.git"  "${PYENV_ROOT}/plugins/pyenv-which-ext"
+checkout "${GITHUB}/pyenv/pyenv.git"            "${PYENV_ROOT}"                           "${PYENV_GIT_TAG:-master}"
+checkout "${GITHUB}/pyenv/pyenv-doctor.git"     "${PYENV_ROOT}/plugins/pyenv-doctor"      "master"
+checkout "${GITHUB}/pyenv/pyenv-installer.git"  "${PYENV_ROOT}/plugins/pyenv-installer"   "master"
+checkout "${GITHUB}/pyenv/pyenv-update.git"     "${PYENV_ROOT}/plugins/pyenv-update"      "master"
+checkout "${GITHUB}/pyenv/pyenv-virtualenv.git" "${PYENV_ROOT}/plugins/pyenv-virtualenv"  "master"
+checkout "${GITHUB}/pyenv/pyenv-which-ext.git"  "${PYENV_ROOT}/plugins/pyenv-which-ext"   "master"
 
 if ! command -v pyenv 1>/dev/null; then
   { echo