Allow customizable date pattern for list page grouping

[henrikje]

When the list.groupByYear parameter is enabled, Congo uses the date pattern 2006 when grouping pages by date.

congo/layouts/list.html

Lines 29 to 30 in 53f7e89

	{{ if $.Params.groupByYear | default ($.Site.Params.list.groupByYear | default true) }}
	{{ range (.Paginate (.Pages.GroupByDate "2006")).PageGroups }}

I do want my pages to be grouped, but grouping only by year is too course-grained. If you've written more than pagination.pagerSize posts the current year, it would basically put all the (visible) posts under the same group which is a bit pointless.

On my personal blog, I've changed that date pattern to January 2006 which gives me some 4-6 posts per group given my weekly publishing pace. I think this gives not only some context to the reader, but also breaks a monotonous list page into nice sections.

I've currently solved this the by making a local copy of list.html and make the change, which isn't ideal. I would love for Congo to support his natively.