Inline command invocation with weep credentials

[patricksanders]

Add a command to let users call a command using the credentials provided by weep.

Example usage:

# this command would get credentials for test_role then call `aws sts get-caller-identity` 
# with the credentials set as environment variables
weep cmd test_role aws sts get-caller-identity

# this command would get credentials for test_role, get assume role credentials for other_role,
# then call `aws sts get-caller-identity` with the credentials set as environment variables
weep cmd test_role --assume-role arn:aws:iam::123456789012:role/other_role aws sts get-caller-identity