Show a warning when a resource path does not start in a slash

[kylef]

This is really similiar to #350.

Repro:

$ echo '# My Resource [realativePath]' | drafter
element: "parseResult"
content:
  -
    element: "category"
    meta:
      classes:
        element: "array"
        content:
          -
            element: "string"
            content: "api"
      title:
        element: "string"
        content: "My Resource [realativePath]"
    content: []

OK