Add metadata to TemplatedEmail

[PetitCassoulet]

No description provided.

[codecov-commenter]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   97.56%   97.56%           
=======================================
  Files          10       10           
  Lines         410      410           
=======================================
  Hits          400      400           
  Misses          5        5           
  Partials        5        5           

Impacted Files	Coverage Δ
templates.go	100.00% <ø> (ø)

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[PetitCassoulet]

Hi @keighl, the metadata field has already been merged for the regular email but is still absent for the templated email, hence my pull request :)