Dont add to ssh-agent unless specified #27

I would suggest implementing a parameter that allows stopping bless from adding the Identity file to ssh-agent.
Namely this lines: https://github.com/lyft/python-blessclient/blob/master/blessclient/client.py#L594-L597

Given a config like:

Match exec "env | grep -q BLESS_COMPLETE || /Users/stype/blessclient/blessclient.run --gui --host '%h'"
	IdentityFile ~/.ssh/blessid

this is not required and could interfere with other credentials.

I think this is required when doing a wrapper around the ssh command but in that case its easy to toggle on in the script.

If this is accepted I can create a PR to cleanup/implement this.