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.
