Postpone engine initialization

[kylekatarnls]

Fix #43

[codecov]

Codecov Report

Merging #51 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #51   +/-   ##
=======================================
  Coverage       100%   100%           
- Complexity      195    201    +6     
=======================================
  Files            13     13           
  Lines           516    534   +18     
=======================================
+ Hits            516    534   +18

Impacted Files	Coverage Δ	Complexity Δ
src/Pug/Symfony/Traits/Options.php	100% <100%> (ø)	10 <0> (+4)	⬆️
src/Pug/PugSymfonyEngine.php	100% <100%> (ø)	41 <0> (+1)	⬆️
src/Pug/Twig/Environment.php	100% <100%> (ø)	31 <11> (-1)	⬇️
src/Pug/Symfony/Traits/HelpersHandler.php	100% <100%> (ø)	58 <12> (+2)	⬆️

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