Support keepalive for REPL instances

[djspiewak]

Certain REPL executables (ahem scala) have significant startup overhead. It would be extremely convenient if we could configure (probably using worksheet-settings for that particular language) a REPL type to remain resident even after an eval. Keepalive would need to be configured both as a flag enabling its function and as a special command to pass to the REPL instance that resets its state (e.g. :reset for scala). We would also need a new command to manually bounce the resident instance if something gets screwed up.