Add ability to use user-specified circuit configuration

[rpalakkal]

Currently halo2-repl always starts off with the DEFAULT_CIRCUIT_CONFIG when running a circuit (halo2-repl) and then tunes from there.

We should add the ability to specify your own configuration from the halo2-repl frontend (perhaps there could be some sort of "Settings" modal) to replace DEFAULT_CIRCUIT_CONFIG.

Related to #20 (so that the config the user inputs is not further auto tuned).