Use persistent: true when persisting env with Application.put_env/3.
Made optional Poison dep more permissive.
Added mix smuggle encode task. Fixed handling of config values
that aren't keyword lists but look almost like them.
t:error_reason is no longer string-typed, instead being one of
:bad_input, :bad_key, :bad_value, or :load_error.
Test coverage improved.
First release, consisting of apply/1, decode/1, encode/1,
encode_file/1, and encode_statement/1.