Update links in frontmatter

[bensheldon]

notfoundbot is fantastic. Thank you!

I would like it to also find and update links in frontmatter too, being able to support:

• when the link is embedded within other text in value of a frontmatter key
• when the frontmatter value is singly a link

I dunno it it makes sense to make this optional, or even to require an allowlist of frontmatter keys to upon which to enable it. My YAGNI proposal would be to implement with no optionality and simply have this part of the baseline behavior.