RFC: Add basic console as proof of concept

If you're using the cli frequently, you have to deal with the boot up time of the interpreter, loading of libraries etc that adds a good 1.5s to each command, at least on my machine. So I thought it might be useful to have a console command that will just allow us to maintain a rdio-cli session open and skip that boot time.

For example, without console:
```
$ time rdio help
-- output --
rdio help  1.45s user 0.13s system 99% cpu 1.581 total

$ time rdio current
-- output --
rdio current  1.75s user 0.22s system 85% cpu 2.290 total
```

With console:
```
$ rdio c
> help
-- output --
0.010000   0.010000   0.020000 (  0.015161)
> current
-- output --
0.000000   0.010000   0.420000 (  0.766850)
````

Benchmarking code left in so you can give it a try yourself.