using _append_path function on plugins

plugins/node/node.load

 ### Main program
-if test -d /usr/local/share/npm/bin
-  set PATH /usr/local/share/npm/bin $PATH
-end
+_append_path /usr/local/share/npm/bin
 
 set PATH ./node_modules/.bin $PATH

plugins/plenv/plenv.load

-if test -d $HOME/.plenv/bin
-  set PATH $HOME/.plenv/bin $PATH
-end
-
-if test -d $HOME/.plenv/shims
-  set PATH $HOME/.plenv/shims $PATH
-end
+_append_path $HOME/.plenv/bin
+_append_path $HOME/.plenv/shims

plugins/pyenv/pyenv.load

 if test -n "$PYENV_ROOT"
-  if test -d $PYENV_ROOT/bin
-    set PATH $PYENV_ROOT/bin $PATH
-  end
-  if test -d $PYENV_ROOT/shims
-    set PATH $PYENV_ROOT/shims $PATH
-  end
+  _append_path $PYENV_ROOT/bin
+  _append_path $PYENV_ROOT/shims
 else
-  if test -d $HOME/.pyenv/bin
-    set PATH $HOME/.pyenv/bin $PATH
-  end
-  if test -d $HOME/.pyenv/shims
-    set PATH $HOME/.pyenv/shims $PATH
-  end
+  _append_path $HOME/.pyenv/bin
+  _append_path $HOME/.pyenv/shims
 end

plugins/python/python.load

-if test -d /usr/local/share/python
-  set PATH /usr/local/share/python $PATH
-end
+_append_path /usr/local/share/python
 

plugins/rbenv/rbenv.load

 if test -n "$RBENV_ROOT"
-  if test -d $RBENV_ROOT/bin
-    set PATH $RBENV_ROOT/bin $PATH
-  end
-
-  if test -d $RBENV_ROOT/shims
-    set PATH $RBENV_ROOT/shims $PATH
-  end
+  _append_path $RBENV_ROOT/bin
+  _append_path $RBENV_ROOT/shims
 else
-  if test -d $HOME/.rbenv/bin
-    set PATH $HOME/.rbenv/bin $PATH
-  end
-
-  if test -d $HOME/.rbenv/shims
-    set PATH $HOME/.rbenv/shims $PATH
-  end
+  _append_path $HOME/.rbenv/bin
+  _append_path $HOME/.rbenv/shims
 end