Previously, any sort of "rich" markup for a definition list's label
would cause the Markdown parser to not recognize a definition list:
```ruby
md = <<~md
`one`
: This is a definition
md
doc = RDoc::Markdown.parse(md)
doc # => [doc: [para: "<code>one</code>\n: This is a definition"]]
```
This commit tweaks the grammar for Markdown definition lists so that
labels can include "rich" markup such as bold (`**`), code (```), etc:
```ruby
md = <<~md
`one`
: This is a definition
md
doc = RDoc::Markdown.parse(md)
doc # => [doc: [list: NOTE [item: ["<code>one</code>"]; [para: "This is a definition"]]]]
```
The [PHP Markdown Extra][1] Spec does not seem to specify whether or not
this should be allowed, but it is allowed in the RDoc format:
```ruby
rdoc = <<~rdoc
+code+::
This is a definition
rdoc
doc = RDoc::Markup.parse(rdoc)
doc # => [doc: [list: NOTE [item: ["+code+"]; [para: "This is a definition"]]]]
```
so accepting this change increases the parity of the two formats.
[1]: https://michelf.ca/projects/php-markdown/extra/#def-list8f943bbba4
08961ce8e3