Themedd 1.1: Introduce hooks in themedd_header function

[arraypress]

I am aware 1.1 will probably not get a public release, but regardless felt it was worth mentioning (in the process of building a site myself using it).

Unlike the original Themedd release, the themedd_header function contains no action hooks throughout and it would save a lot of code duplication to have a few included at strategic places throughout that function.