Providing ability to use concourse metadata from within params input files

[ballad89]

Hello

Started using your resource and made some changes to it and thought I should push it upstream

• ability to use concourse metadata in email text like ${BUILD_ID}, etc
• made source.to optional if params.additional_recipient is set, this gives you the ability to determine who to send the email to at build time from a git commit for example

[cf-gitbot]

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[alexander-matsievsky]

@ballad89 did a great job!
@rosenhouse, @mmb could you take a look?
Very much appreciated!

[alexander-matsievsky]

@ballad89 Would you be interested in that concourse/concourse#1434?

[alexander-matsievsky]

Unfortunately, the fork is the only option,
concourse/concourse#1434.