Add aws-vault export cmd

[mtibben]

The exec --json flag was always a bit weird, because the --json flag changes the behaviour and doesn't "exec" anything at all.

This remedies that weirdness by creating a aws-vault export command that exports credentials in a variety of formats.

• aws-vault export --format=env PROFILE prints in env-var format e.g. FOO=BAR
• aws-vault export --format=export-env PROFILE prints in env-var format with en export e.g. export FOO=BAR
• aws-vault export --format=json PROFILE prints in json format compatible with credentials_process
• aws-vault export --format=ini PROFILE prints in ini format compatible with ~/.aws/credentials

The old exec --json flag still works but also prints out a deprecation notice to stderr

Fixes #761 #816