load inputrc after IPython readline config

There is some subtlety, in that *user* IPython config should be higher priority than inputrc (more specific), but *default* IPython config should be lower (less specific).

If readline config is customized, it will be allowed to run on libedit, trusting the user to know what she is doing.

closes #5915
I don't think so, but it's arguable. libedit *is* configurable, but its syntax is slightly different. In master, we prevent users from explicitly configuring libedit because loading readline config was only one action, whether its IPython's defaults or not. After this PR, a user with libedit can configure libedit explicitly, but it will still not load our defaults, which are only for readline.