Allow layout components to be grouped together

[fehnomenal]

trafficstars

Fixes #708 by trying to load src/routes/_layout.{ext} and src/routes/_layout/index.{ext} before falling back to the default layout.

Works locally by using npm link.

[benmccann]

Thanks for the suggestion. We probably don't want to add any new features to Sapper that aren't in SvelteKit because we don't want people to lose functionality when upgrading. It'd probably be best to raise the idea there and see what people think of it

[benmccann]

SvelteKit 1.0 is now out and Sapper is deprecated, so I'm going to close this