Add fix_string() to the API

[seanthegeek]

I saw that the CLI has a fix option, but that is not currently exposed in the API.

I'm creating a Python project that uses html2text to convert some HTML content to Markdown. The results are pretty messy. The markdownlint extension for Visual Studio Code can clean up a lot of linting errors automatically. I'd like to do the same thing before writing the Markdown content to a file to save time and effort.