Support for rendering Markdown as HTML

[Badgerati]

This is to add in support for rendering Markdown files as HTML content - using the PowerShell function: ConvertFrom-Markdown.

There should be:

• A new markdown function, like html, that takes raw data or a file path and converts the markdown to html. At the end of the function, it should be similar to the html function when render the content back as html.
• Internal support for markdown as a view engine. Currently there is pode, html, and anything else for custom/third-party - this will add inbuilt support for a new markdown type, which will get markdown files and convert them to html.
• Could have possible support on static content; so if it sees a .md/.markdown file, it automatically converts it to html 🤔
• The markdown mimetype will need adding to the Get-PodeContentType function.

[Badgerati]

On-hold, pending PowerShell/PowerShell#9390 (comment) (issue with running the ConvertFrom-Markdown command in a new runspace)

[Badgerati]

PowerShell/PowerShell#9390 (comment) has been fixed, just hoping it gets into v6.2.1

[Badgerati]

Looks like v6.2.1 is released (https://github.com/PowerShell/PowerShell/releases/tag/v6.2.1), which also comes with the Markdown fix!

Once it's released as an installer/container I'll continue with this.

[Badgerati]

It would appear that v6.2.1 actually contained security fixes, and not what's on the release notes for the tag.

Guess this is waiting till v6.2.2.

[Badgerati]

Can finish this off now, with rendering Markdown as HTML only been supported in PS7+.