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'] ]
+!}

tests/fixtures/theme-mod-location/etc/mkdocs.yml

@@ -0,0 +1,15 @@
+site_name: syspath inclusion
+docs_dir: ../refsite # dummy
+site_dir: ../site
+theme:
+  name: readthedocs
+  custom_dir: doc_theme/
+plugins:
+- semiliterate:
+    ignore: [refsite]
+    merge_docs_dir: false
+    include: []
+    semiliterate:
+      - pattern: '[.](base).generator$' # Amend readthedocs theme
+        destination: '\1.html'
+        ensurelines: false