add a pterm formatter

[evanmcc]

This PR adds a formatting command {pterm, key, DefaultVal}.

This is meant for adding global things to the formatting string, which can be set once, on startup, rather than repeatedly stuffed into the pid-specific metadata. My particular use case is the OS pid, so it's easier to tell when, in the logs, a particular process restarted.

[jadeallenx]

The only hesitation I have is that persistent_term was introduced in OTP 21.2, so the test should skip if OTP is less than that. But I can fix that up

[jadeallenx]

closing in favor of #530