fix: Interpret custom_dir relative to current directory, not config directory

The custom theme dir handling was not working in case the mkdocs config
  file was not in the top-level directory. Since mkdocs itself seems only
  to work in the top-level directory, this PR modifies mkdocs_semiliterate
  to interpret custom theme dirs relative to the current directory.

  Also, improves the harmonization between current mkdocs_simple_plugin
  code and this code.

  Resolves #28.

tests/fixtures/theme-mod-location/etc/doc_theme/.base.generator

@@ -0,0 +1,4 @@
+{! "\syspath mkdocs/themes/readthedocs/base.html"
+extract:
+  replace: [ ['([<]body.*$)', '\1\n<p>Kilroy was here.</p>\n'] ]
+!}