[MlMt] Importance: header.

[Scott A. McIntyre]

One more question:



> Such headers are badly standardized, but if you know what works for 
> the recipient then you can add a custom header as described 
> [here](http://manual.mailmate-app.com/hidden_preferences#composing). 
> Look for `MmAdditionalComposerHeaders`. The example uses `X-Priority` 
> which is another non-standard priority header.


Is it possible to also set a default value?  I see that it supports 
headerName and type, but does it also support a value, or any other 
parameters?

Ta,
Scott