allow setting a custom HISTFILE before oh-my-zsh is loaded

lib/history.zsh

@@ -1,5 +1,7 @@
 ## Command history configuration
-HISTFILE=$HOME/.zsh_history
+if [ -z $HISTFILE ]; then
+    HISTFILE=$HOME/.zsh_history
+fi
 HISTSIZE=10000
 SAVEHIST=10000