checking plugins paths before adding them to PATH

plugins/node/node.load

@@ -1 +1,3 @@
-set PATH /usr/local/share/npm/bin $PATH
+if test -d /usr/local/share/npm/bin
+  set PATH /usr/local/share/npm/bin $PATH
+end

plugins/python/python.load

@@ -1 +1,3 @@
-set PATH /usr/local/share/python $PATH
+if test -d /usr/local/share/python
+  set PATH /usr/local/share/python $PATH
+end

plugins/rbenv/rbenv.load

@@ -1,3 +1,9 @@
-set PATH $HOME/.rbenv/bin $PATH
-set PATH $HOME/.rbenv/shims $PATH
+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
+
 rbenv rehash >/dev/null ^&1