allow msg to accept a function to customize output string

[brendancwood]

I'm not a fan of the msg being a string that's bound by lodash {{}} templating, so why not allow msg to accept a function so I can manipulate rec/res how I want with js and return a string however I want?

[brendancwood]

also, should I make the error logger do the same thing? I think I may do that here in another commit

[brendancwood]

@bithavoc we're using this lib for logging and this pr would make it so we don't have to use my personal fork of it, let me know if there's any other feedback, would be happy to address anything

[bithavoc]

@brendancwood sure, can you fix the conflicts so I can merge? Thanks.

[brendancwood]

done, let me know if you need anything else thanks!

[bithavoc]

@brendancwood excellent, forgot to mention it, but the Readme should contain some help instructions on how to use this new functionality. can you add it?

[brendancwood]

hopefully what i added to the readme is clear enough

[bithavoc]

Thanks ✌️