Implement strict and assumeObject modes

Causes templates to throw when lookup fields are not defined within the
context. Strict mode will throw when any field is omitted. assumeObjects mode
maintains the existing behavior of outputting an empty response when fields
are not defined but allows for performance/size optimizations by not doing
safety checks for intermediate objects in the lookup chain.

Strict mode effectively disables the helperMissing and inverse handling for
blockHelperMissing as templates will throw rather than call those features for
missing data fields.

Fixes #651 Fixes #805