Feature: Sortable contents

[rathesDot]

Currently the content pages stored in the same directory have no specific sort order. But there should be a way so that the Menu implemented in #2 can be used to render the menu in a desired order.

The first idea is to simply add a prefix to the filenames: 1-index.md is the first page and 2-test.md is the second page ... Therefore one must update the RouteResolver, but you would not need any extra configuration file.

[rathesDot]

In #18 we refactored to use directories. So now, instead of renaming the file, we would rename the directory.

We should make sure that the separator (in the example above the -) is configurable.