Consider replacing `<pre>...</pre>` tags since Nagios strips out `<` and `>` characters

[atc0005]

While reviewing and responding to an internal ticket, I was reminded that Nagios strips out characters defined in the illegal_macro_output_chars setting as illegal or dangerous:

illegal_macro_output_chars=`~$^&"|'<>

This prevents this:

https://github.com/atc0005/go-nagios/blob/8007ac77bae39dcef015975e9d49b74d53aba92c/nagios.go#L150-L161

from working as intended. I'll need to use another approach here.

refs:

• https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/3/en/configmain.html#illegal_macro_output_chars
• https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/4/en/configmain.html#illegal_macro_output_chars