[FEATURE ember-handlebars-compiler-ast-to-precompile] Allow precompile to accept an already existing Handlebars AST

[FEATURE pass-ast-to-precompile] Adding feature flags

[FEATURE pass-ast-to-precompile] Changing flag name
Merge pull request #5341 from chadhietala/pass-ast

[FEATURE pass-ast-to-precompile] Allow precompile to accept AST