Add support for sorting on Hugo weight

[mrled]

Hello,

I added a sorting method based on weight in the frontmatter of Hugo markdown files.

Weight is a concept in Hugo that affects the order of items in a list. Normally items might be ordered alphabetically or by creation date, and weight allows overriding that order.

This is just a proof of concept. It only supports Markdown files with .md suffix and YAML frontmatter. I'm interested in taking this further depending on feedback.

A future version could improve by:

• Supporting all of Hugo's frontmatter types, including TOML and JSON
• Supporting all of Hugo's content formats (possibly starting with just Markdown and HTML which are built in) and file extensions (markdown, md, mdown, htm, html)
• Allow sorting by content date, which is especially useful for blogs; a robust implementation would understand multiple types of dates
• Parsing frontmatter via a real parser instead of regex - although I'm torn on this, as real parsers have the disadvantage of requiring libraries.