Correctly match `-#` (caption)

[Kyrela]

Should match -# elements (aka captions) as a new node type.

For example:

Foo
-# bar **baz**

Is rendered as:

And should be parsed as:

[
    {'node_type': 'TEXT', 'content': 'Foo\n', 'children': []},
    {'node_type': 'CAPTION', 'content': 'bar **baz**', 'children': [
        {'node_type': 'TEXT', 'content': 'bar ', 'children': []},
        {'node_type': 'BOLD', 'content': 'baz', 'children': [
            {'node_type': 'TEXT', 'content': 'baz', 'children': []}
        ]}
    ]}
]