Easier access to parsed headers

[pacak]

I'm trying to use comrak as a markdown parser and transformer and current goal is to extract headers along with their levels.

Finding headers is easy - pattern matching on values and taking NodeValue::Heading(_). Extracting the text is not. Header contents are stored in a private field content and to access it I have to do something like:

1. use format_commonmark to render the whole node to a vector of bytes (allocates)
2. use String::from_utf8 to get back a string (contains unwrap/expect that should never fire but it's still there)
3. trim # and \n and convert to owned string (allocates again)

Ideally I would like to be able to access content field directly.

[kivikakk]

Header contents are not, strictly speaking, stored in a private field content. The field is private precisely because it is not a sensible user-facing value. See the last time this came up: #175 (comment).