Optional warning for orphaned files

[kevgo]

When working on isolated documentation without inbound links I want to be notified about files that aren't linked to from within the documentation So that my documentation does not accumulate hidden cruft and all files are relevant.

• if the roots option lists files, all other files that are not linked to cause an error
• this checks Markdown and image files
• the configuration file allows to list files that can be orphaned, like licenses etc

Scenario: orphaned file
  Given a runnable file "1.md"
  And my source code contains the file "foo.png"
  When trying to run "text-run --no-orphans"
  Then the test fails with:
    | FILENAME      | foo.png                  |
    | ERROR MESSAGE | file foo.png is not used |
    | EXIT CODE     | 1                        |