add support for “sidecar” frontmatter at the destination url of proxied resources

a “cheap” way to support adding data to pages created through `proxy` or `page` by adding frontmatter at the expected destination url.

for example proxying a page to `/proxied.html`, by adding a sidecar frontmatter file at `source/proxied.html.frontmatter` (or source/proxied.frontmatter if directory indexes is activated) the frontmatter will be merged with the proxy source and available to `current_resource` at the destination.
Merge pull request #1414 from stevenosloan/feature/support_sidecar_frontmatter_for_proxied_resources

add support for “sidecar” frontmatter at the destination url of proxied resources