Allow extensions to take extra options

It would be great if extensions could take extra arguments from the command line. Maybe everything after a `--` could be passed directly to the extension, unparsed by timew?

Start by tracing how command-line arguments are parsed and how extensions are invoked. Determine how arguments after `--` should be separated from Timewarrior options and passed through unchanged; the issue does not name specific files or tests, so inspect the CLI and extension entry points first.

Written by the indexing model from the issue text.