Add overrides for source_profile and mfa_serial to add-profiles subcommand

[chrisgilmerproj]

The previous iteration of the setup-new-aws-user add-profile subcommand relied on copying values for source_profile and mfa_serial from an existing AWS profile in ~/.aws/config. This is not ideal when you wish to customize one or the other or both of those values. Now you can run this command in several ways to get the desired output:

With only the existing AWS profile provided:

go run ./cmd/ add-profile --aws-profile trussworks-org-root --iam-role OrganizationAccountAccessRole --aws-profile-account trussworks-cgilmer:123456789012

With one of either source-profile or mfa-serial provided along with the existing AWS Profile:

go run ./cmd/ add-profile --aws-profile trussworks-org-root --source-profile trussworks-org-root --iam-role OrganizationAccountAccessRole --aws-profile-account trussworks-cgilmer:123456789012

WIth both the source-profile and mfa-serial names provided:

go run ./cmd/ add-profile --source-profile trussworks-org-root --mfa-serial arn:aws:iam::123456789012:mfa/cgilmer.org-root --iam-role OrganizationAccountAccessRole --aws-profile-account trussworks-cgilmer:123456789012

I plan to release this as v0.5.2.

[eeeady]

Looks good to me. Works locally and doesn't break any existing tests.