split percol.load into README and percol_select_history.fish

plugins/percol/README

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+Please bind percol_select_history in your favorite key.
+
+If you'd like to bind CTRL+R
+
+function fish_user_key_bindings
+  bind \cr percol_select_history
+end

plugins/percol/percol_select_history.fish

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+function percol_select_history
+  history|percol|read foo
+  if [ $foo ]
+    commandline $foo
+  else
+    commandline ''
+  end
+end