add `Config.getlist` method
refactor markup and sanitization code

This commit introduces a new configuration section [markup] to refine
Misaka's Markdown extensions (by default strikethrough, superscript and
autolink).

Furthermore, you can set custom HTML elements/attributes that are
allowed, e.g. to enable images, set

  [markup]
  allowed-elements = img
  allowed-attributes = src

The refactorization separates HTML sanitization from Markdown -> HTML
and allows to include new markup languages such as BB Code or
reStructuredText.
add tests for 81ecc8e
document new [markup] section
expand db path, closes #68
Merge branch 'feature/configurable-markdown', closes #62