submark: Extract a part from CommonMark/Markdown docs

submark is a CLI program to extract some particular section from a givne CommonMark/Markdown document. I use it for myself to extract the latest version section from the CHANGELOG.md file, and then reuse the text for the corresponding release note on GitHub releases, during automated release process which is run on CI.

Usage examples

The following examples use John Gruber's original Markdown introduction:

$ wget https://daringfireball.net/projects/markdown/index.text

Extracting an H3 heading Command-Line:

$ submark --h3 "Command-Line" index.text
### Command-Line

Use the `--html4tags` command-line switch to produce HTML output from a
Unix-style command line. E.g.:

    % perl Markdown.pl --html4tags foo.text

Type `perldoc Markdown.pl`, or read the POD documentation within the
Markdown.pl source code for more information.

Note that its textual style differ from the original text. Since submark internally builds an abstract syntax tree from the given input text and then render again the result tree, it doesn't maintain trivial styles (e.g., whitespaces, ATX vs. Setext headings) but only the semantics.

There are options from --h1 to --h6:

$ submark --h2 "Download" index.text
## Download

[Markdown 1.0.1](http://daringfireball.net/projects/downloads/Markdown_1.0.1.zip)
(18 KB) -- 17 Dec 2004

The leading heading can be omitted:

$ submark --h2 "Download" --omit-heading index.text
[Markdown 1.0.1](http://daringfireball.net/projects/downloads/Markdown_1.0.1.zip)
(18 KB) -- 17 Dec 2004

By Unix convention, - means pipe:

$ submark --h2 "Download" - < index.text
## Download

[Markdown 1.0.1](http://daringfireball.net/projects/downloads/Markdown_1.0.1.zip)
(18 KB) -- 17 Dec 2004

There's -o/--out-file option as well:

$ submark -o download.md --h2 "Download" index.text