Change footer filter to allow more styling

[salcode]

@bryanwillis brings up that by switching from genesis_footer_creds_text to genesis_footer_output we gain more control over modifying the footer area.

See PR #110

My first instinct is as a starter theme, we should stick with genesis_footer_creds_text since we're only changing the text with the understanding that others modifying the theme will most likely use genesis_footer_output because they'll want to make more changes.

As I think about Bryan's point, if I'm modifying the theme it would be easier to already have the filter I want to use and just modify it rather than having to determine the filter I want is genesis_footer_output.

In addition to the filter change, this PR centers the Footer Credentials, which is not an option when using genesis_footer_creds_text.

I am a little concerned about the edge case where someone is already applying a filter to genesis_footer_creds_text and then they update. I'd hate to see their Footer Credentials revert to the project Footer Credentials.

Maybe we should keep genesis_footer_creds_text and add a second call which goes to genesis_footer_output and adds the <p class="text-center"> tag around it, thought perhaps this is over-engineering.

Okay, this ended up being a brain dump. Let's give this a little thought and decide how to handle it. I'd love to hear input if others have thoughts.

[salcode]

I'm seeing a lot of designs come in with full width footers.

Since we're applying .container on the footer element this is not optimal. I think we should drop the .container class on footer and use the genesis_footer_output hook to write on our footer (which would include the tag with the .container class).