Extend header template to support new front matter configuration settings

Following settings have been created to add styling of the site header:1
1. `bg_style` - CSS styling of background image on header (e.g. can control positioning of background image)
2. `mask_style` - CSS styling of the overlay mask (e.g. can control the opacity of the mask)
3. `title_style` - CSS styling of the header text (e.g. can be used to modify the minimum height of the text block)