Allow for filtering of generator specific tags in markdown

[mdales]

Currently when we generate the searchable stems from markdown, in my Hugo sites all the custom tags (aka shortcodes) will get picked up. For example I have one that does images:

# My nice post

Here is an image:

{{< figure IMG_1234.jpeg >}}

Wasn't that nice?

Hugo also allows for enclosing shortcodes:

{{< mycodeblockwrapper >}}
    if astute:
        print("Why isn't he using the markdown code wrapper in his blog?")
{{</ mycodeblockwrapper>}}

I'd like to add a feature to the FrontMatterDocument struct that lets you provide a filter for non-standard tags like this for when it's generating the plainText from the markdown. I suspect the neatest way to do that is way for the Swift Regex library to leave beta, and then allow people to provide a custom set of regexs to the document constructor that will be run on the text that is extracted from the markdown.