Support alternative definition list syntax

Support should be added for the alternative syntax of [definition lists](https://textile-lang.com/doc/definition-lists). Textile-lang.com gives the following example:

```
; HTML
: HyperText Markup Language, based on SGML.
; XHTML
: HTML 4.0 rewritten to be compliant with XML rules.
; HTML5
: The latest revision of the HTML standard.
: Still under development.
```