implement saved history for consoles

[stevengj]

The notebook doesn't seem to use the history_request mechanism yet, but it is used in the qtconsole. Currently, we have a stub implementation that saves no history, but it would be good to implement a saved-history file if people are going to use the console.

I was thinking of using an implementation and file format similar to what @loladiro is experimenting with in his REPL.jl module. On the other hand, IPython uses an SQLite database, which seems like overkill for a sequential list of strings but I guess is nicer if multiple processes want to write history?

IPython technically requests the text/plain output from the history too, as @minrk explained in ipython/ipython#3806. I'm not sure why, though—it doesn't seem like qtconsole and I was thinking of just saving the input strings and sending empty output strings to IPython.