Add 'stdin' and 'stdout' flags to 'import' and 'export' commands

[nikoksr]

Is your feature request related to a problem? Please describe. Class configs should be im- and exportable via stdin and stdout. It should push raw TOML configs into stdout and accept raw TOML configs from stdin.

Describe the solution you'd like The flags should work and be implemented similarly to the other import flags (config, directory, remote-repo).

Example

proji class import --stdin <raw class config here>
proji class export --stdout <class label>