Added helper module for defining configurables (e.g. syntax highlighters, math engines, ...)
Syntax highlighters are configurable now

* Syntax highlighters can now be set via the syntax_highlighter
  options which defaults to coderay for backwards-compatibility

* Options for the syntax highlighter can be set with the
  syntax_highlighter_opts option

* The old coderay_* options are deprecated as of now and will be
  removed in 2.0.0

* The coderay implementation has been extracted and is now just
  another syntax highlighter implementation.
Documentation: Added #id to all options for referencing