Allow configuring how to build the page

[findmyway]

Currently, we have a build_page function to merge the head, content and foot into a page:

build_page(head::String, content::String, pgfoot::String, foot::String) =
    head * html_div(locvar("div_content"), content * pgfoot) * foot

Though the class attribute is customizable, one may also need to set other attributes, or even change the node name.

A not so aggressive solution is to change the html_div function into html_hk, like https://github.com/tlienart/Franklin.jl/pull/519/files

Or can we just provide a whole configurable page like below?:

// page.html
{{insert head.html}}
{{ content }}
{{ insert pgfoot.html }}
{{ insert foot.html }}

[findmyway]

Closed with #521