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.

mkdocs_semiliterate/plugin.py

@@ -320,7 +320,7 @@ terminate: '^\s*\)'
         #    `semiliterate` still incorporates all standard Markdown files
         #    because of the following `extract_standard_markdown` parameter.
         ('extract_standard_markdown',
-         config_options.Type(dict, default={}))
+         config_options.Type(dict, default={})),
         #    If the `enable` key of this dict parameter is true
         #    (it defaults to the opposite of `copy_standard_markdown`),
         #    it adds a semiliterate block causing extraction (and hence
@@ -333,6 +333,10 @@ terminate: '^\s*\)'
         #    (which is also the default when `copy_standard_markdown` is true)
         #    will prevent automatic extraction (and hence disable
         #    inclusion-directive processing) from standard Markdown files.
+        ('extract_on_copy',
+         config_options.Type(bool, default=False)),
+        #    Whether to also attempt extraction from a file that is copied
+        #    verbatim (because of matching the `include_extensions`).
     )
 
     def on_config(self, config, **kwargs):
@@ -365,6 +369,8 @@ terminate: '^\s*\)'
     def extract_from(self, from_directory, name, to_directory):
         if any(ext in name for ext in self.exclude_extensions):
             return False
+        if not self.config['extract_on_copy'] and self.in_extensions(name):
+            return False
         return super().extract_from(from_directory, name, to_directory)