Most e-mail user agents (MUA) consider text in angle brackets as a hyperlink, even and especially in plain text e-mails. So for most cases, just wrap the URL in angle brackets (like this: "bla bla bla - klick this link: <http://www.example.com/once/upon/a/time/there/was/a/file/burried/deep/inside/the/filesystem.txt> bla bla bla") and everything else happens automatically. No need to generate HTML. A properly secured MUA does not use a fully-featured HTML renderer with CSS and JS support, anyway.

Alexander