Magic Folder: add magic folder join commandline tool

[dawuud]

For "tahoe magic-folder join INVITATION LOCAL_DIR" :

1. Parse INVITATION as COLLECTIVE_READCAP|CLIENT_WRITECAP.
2. Write CLIENT_WRITECAP to the file magic_folder_dircap under the client's private directory.
3. Write COLLECTIVE_READCAP to the file collective_dircap under the client's private directory.
4. Edit the client's tahoe.cfg to set [magic_folder] enabled = True and [magic_folder] local.directory = LOCAL_DIR.