Add optional logo to header.yml

[Scott Weber]

Issue Description:
Add the ability to specify a header logo image in the header.yml data file with a URL. https://github.com/18F/uswds-jekyll/blob/master/_data/header.yml

Adding a logo should be optional and if not wanted could be commented out from header.yml.

See https://oes.gsa.gov/ for an example of a logo in the header and see the following component guide for the different header types https://components.designsystem.digital.gov/components/detail/header--default.html

[Eddie A Tejeda]

Hi @rahearn, what's the status on this?

[Ryan Ahearn]

@eddietejeda I wasn't able to merge #170 until it had an approving review. Merged now.