v0.10.0: textual healing

breaking changes

• --tag has been renamed to --section-tag

new features

• --section-path has been added; if specified, the --in section will be
  loaded and then the given section path will be loaded and rendered with the
  --in section as its context.

  this is useful for things like release notes where the release notes aren't
  included into the main docs section but need it for resolving references.

• a basic text renderer has been added, using text/template instead of
  html/template and only including a few non-markup-specific templates. this
  is useful as a base for Markdown renderers.

• the joinLines function previously added in v0.9.0 has been moved to the
  new text renderer.

misc

• the 'parsing section' log line has been switched to debug level.