add goenv plugin

plugins/available/goenv.plugin.bash

@@ -0,0 +1,18 @@
+cite about-plugin
+about-plugin 'load goenv, if you are using it'
+
+# Don't modify the environment if we can't find the tool:
+# - Check if in $PATH already
+# - Check if installed manually to $HOME
+_command_exists goenv ||
+  [[ -x "$HOME/.goenv/bin/goenv" ]] ||
+  return
+
+# Set GOENV_ROOT, if not already set
+export GOENV_ROOT="${GOENV_ROOT:-$HOME/.goenv}"
+
+# Add GOENV_ROOT/bin to PATH, if that's where it's installed
+[[ -x "$HOME/.goenv/bin/goenv" ]] && pathmunge "$GOENV_ROOT/bin"
+
+# Initialize goenv
+eval "$(goenv init - bash)"