add support for dynamic template data

[nickhudkins]

Resolves #260

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Adds support for Dynamic Template Data via the Mail Helper

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #262 into master will increase coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   70.75%   71.61%   +0.85%     
==========================================
  Files           3        3              
  Lines         448      451       +3     
==========================================
+ Hits          317      323       +6     
+ Misses        127      123       -4     
- Partials        4        5       +1

Impacted Files	Coverage Δ
helpers/mail/mail_v3.go	99.24% <100%> (+1.15%)	⬆️

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[thinkingserious]

Solved with: #263

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