Add possibility to alter subject before print it

[dmpvost]

this should allow us to update the current subject with .roarr.js / remove/add fields, have some control on the subject.

It's an idea, I don't know if it's the right place, and the right method. I'm not sure to understand everything.

Just a proposal

(not tested)

[coveralls]

Pull Request Test Coverage Report for Build 42

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 39:	0.0%
Covered Lines:	1
Relevant Lines:	1

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💛 - Coveralls