feat: Add extract_on_copy option

Previously, mkdocs_semiliterate would always attempt to extract documentation
  from a file, even if it matched the `include_extensions` pattern for files to
  be copied to the documentation site verbatim.

  Now, by default, such files are not considered candidates for extraction,
  even if they match a semiliterate pattern.

  Adds a configuration option `extract_on_copy` which can be set to `true` to
  restore the prior behavior. Also adds tests for the behavior with and without
  `extract_on_copy` and makes all `mkdocs build` commands in the tests strict,
  which they always should have been.

  Resolves #17.

tests/fixtures/extract-inclusion/refsite/raw.txt

@@ -0,0 +1,7 @@
+This is a free-form file, used as an example
+---
+# Internals
+
+But this is __markdown__ hidden within.
+---
+There can of course be stuff after it.