arcyon task-create: support 'projects' parameter

Add support for specifying which projects to attach a new task to with
the 'arcyon task-create' subcommand.

Test Plan:
$ ./precommit.sh
tried attaching task to a new project
arcyon task-create: fix some typos in messages
arcyon task-update: subcommand for updating tasks

Add a new subcommand 'arcyon task-update' for updating and commenting on
existing Maniphest tasks.

Test Plan:
exercised in
$ ./precommit.sh
tried to update various attributes of existing tasks