The previous default slug-fn used by the slug task assumed
Jekyll-style naming, e.g., "2001-01-01-my-post.md", and if this
assumption was violated it would return non-sensical results.

Previous slug-fn:

| Source filename         | Slug                | Final filename           |
|-------------------------|---------------------|--------------------------|
| "2001-01-01-my-post.md" | "my-post"           | "my-post.html"           |
| "index.md"              | ""                  | ".html"                  |
| "my-great-new-post.md"  | "post"              | "post.html"              |

The new default slug-fn handles Jekyll-compliant names in the same
way, but if given a filename that doesn't fit the Jekyll format it
will just strip the extension.

New slug-fn:

| Source filename         | Slug                | Final filename           |
|-------------------------|---------------------|--------------------------|
| "2001-01-01-my-post.md" | "my-post"           | "my-post.html"           |
| "index.md"              | "index"             | "index.html"             |
| "my-great-new-post.md"  | "my-great-new-post" | "my-great-new-post.html" |