Running add-identities on first SSH run

[eritbh]

We can identify whether identities have been pulled by checking for the existence of the ssh_config file. Make a wrapper that will run the add script only if that file doesn't exist yet, then calls ssh passing through arguments. Users would be able to use the wrapper by adding e.g. alias ssh="op-ssh" to their shell config.

This may not actually need a wrapper; an option could be added to the existing script to do nothing if keys have already been pulled, and it could be used as e.g. alias ssh="op-add-identities -z; ssh" assuming a similar alias structure is available across shells.