Handle Matrix URIs from CLI

[foxyseta]

Other software (e.g., a browser, a PDF reader, or an email client) might try to open a Matrix URI with iamb. I believe iamb's CLI interface on startup should add an optional URI argument specifying an action to be run on startup, according to https://spec.matrix.org/latest/appendices/#uris. Note that this includes both matrix: and https://matrix.to URIs.