Support install locations other than $HOME/.rbenv.

Define RBENV_HOME env variable in libexec/rbenv and let all script delegates use this variable to determine rbenv's install location.

libexec/rbenv-init

@@ -55,9 +55,9 @@ if [ -z "$print" ]; then
   exit 1
 fi
 
-mkdir -p "${HOME}/.rbenv/"{shims,versions}
+mkdir -p "${RBENV_HOME}/"{shims,versions}
 
-echo 'export PATH="${HOME}/.rbenv/shims:${PATH}"'
+echo 'export PATH="'${RBENV_HOME}'/shims:${PATH}"'
 
 case "$shell" in
 bash | zsh )