ENH: Allow disabling Jinja

[Miksus]

Introducing new argument, use_jinja.

[codecov-commenter]

Codecov Report

Merging #40 (7e0cbd7) into master (c410729) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   99.48%   99.49%           
=======================================
  Files          13       13           
  Lines         584      589    +5     
=======================================
+ Hits          581      586    +5     
  Misses          3        3           

Impacted Files	Coverage Δ
redmail/email/body.py	100.00% <100.00%> (ø)
redmail/email/sender.py	99.37% <100.00%> (+<0.01%)	⬆️

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