[enhancement] hakyll.hs is now fitted to hakyll4.

previously hakyll.hs was designed based on hakyll3 version.
As Hakyll4 released, I rework the code to fit to hakyll4.

Current hakyll.hs is able to be compiled with hakyll4 library. Though,
it is rendering a little differently. The First heading of each post
becomes to H2 not H1.
[bug-fix] demoteHeader is removed to show H1 tag.

The header showed as H2 previously. It was caused by demoteHeader
function. This function lowered one header depth for every headers in
the articles.

After removing the line (see the patch), it works fine.
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