Non-arithmetic commands

[coriolinus]

The parser should be able to evaluate commands like help, help FOO, exit, etc. eval_and_print should be able to delegate to appropriate command handlers in that case.

Minimal commands necessary for completion:

• help: enumerate supported functions and operators and a basic syntax description
• quit: exit cleanly. Alternative to ctrl-c.

Bonus objectives:

• help FOO: for any FOO in supported functions and operators, emit a description of what that operation does.
• save PATH: save history (input expressions and results) to PATH
• load PATH: load history (input expressions and results) from PATH
• history: replay entire history