Introduce a way to customize page

[NiccoMlt]

KOMA-script classes are known to be incompatible with fancyhdr package (see #4), which seems to be the de-facto standard choice in many LaTeX guides and forum answers.
KOMA-script classes offer scrlayer to provide control of ‘background’ and ‘foreground’ layers for a page style description.

This template should let the user apply these customizations in some ways.

1. a possible solution would be the usage of a fancyhdr-to-scrlayer adapter like scrlayer-fancyhdr; this solution seems to be still experimental right now.
2. another solution is to use scrpage: in particular, there are two packages:
   • scrpage2, now deprecated, incorporates a set of predefined, configurable page styles and is designed to ease integration into documents using the KOMA-Script suite.
   • scrlayer-scrpage is a official package of the KOMA-script bundle that let the user define and manage page styles by controlling page headers and footers like scrpage2.