fast-import: add option command

fast-import: add option command

This allows the frontend to specify any of the supported options as
long as no non-option command has been given. This way the
user does not have to include any frontend-specific options, but
instead she can rely on the frontend to tell fast-import what it
needs.

Also factor out parsing of argv and have it execute when we reach the
first non-option command, or after all commands have been read and
no non-option command has been encountered.

Non-git options are ignored, unrecognised options result in an error.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>