Dereference error

[mikebridge]

When TemplateContext.ErrorWhenValueMissing() is toggled, an error message is rendered whenever an undefined variable is accessed.

closes #22

[codecov-io]

Current coverage is 90.30%

Merging #32 into master will increase coverage by +0.30% as of 2aa9ee3

@@            master    #32   diff @@
=====================================
  Files          134    134       
  Stmts         4161   4218    +57
  Branches       435    445    +10
  Methods          0      0       
=====================================
+ Hit           3745   3809    +64
+ Partial        161    156     -5
+ Missed         255    253     -2

Review entire Coverage Diff as of 2aa9ee3

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