Add identities command for external MUAs

[elizagamedev]

When replying to a message with an MUA, said MUA should default to using the same sender email address/username as the original addressee. mujmap should implement an identities command which prints a machine readable (json) list of all available JMAP identities, so software can determine the correct identity to reply with.

[elizagamedev]

This should seamlessly support "identities" provided by #30, so printing the raw JSON we get from the server may not be sufficient.