Allow reading date from filename

[jplatte]

Same as #3: Currently using this for blog.turbo.fish, but easy enough to move away from if it turns out hard to fit into Hinoki's design (which shouldn't be the case though).

[jplatte]

I think I'll add something like date = { from = "path_prefix" } or date = { path_prefix = "%Y-%m-%d" } for this.

[jplatte]

Going for templating instead. Done in 196ca64.

[jplatte]

Actually only half done, the filter is there but not usable in date default yet.

[jplatte]

Now it's done!