Allow saving without custom plugin

[ianjennings]

Right now we need to have a custom plugin before any of the server functions work. We shouldn't have to have a custom plugin on the client to save.

@garrettluu says:

figured it out! this bug ONLY happens when we have no custom plugin
so it probably does something weird when trying to encode an empty string

See #93.

[garrettluu]

I believe this is an issue with how REPLState encodes and decodes empty strings; the backend should already be able to handle this. As you mentioned this morning, we can modify REPLState to store raw strings instead of base64 encoding and it will probably fix this issue