Add jsone:term_to_json_string/1 as the default value of map_unknown_value option.

[sile]

This PR makes it possible to encode all Erlang terms by default.

% Before: Cannot encode PID.
> {error, _} = jsone:try_encode(self()).

% After: PID (and other Erlang terms that aren't natively supported by `jsone`) are
%        encoded using the result of `io_lib:format("~p", [...]])`.
> jsone:try_encode(self()).
{ok,<<"\"<0.198.0>\"">>}

If you prefer the previous behavior, you need to specify {map_unknown_value, undefined} explicitly.

Resolves #64

[codecov-commenter]

Codecov Report

Merging #65 (7662c42) into master (f34edbc) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   90.41%   90.44%   +0.03%     
==========================================
  Files           3        3              
  Lines         292      293       +1     
==========================================
+ Hits          264      265       +1     
  Misses         28       28              

Impacted Files	Coverage Δ
src/jsone_encode.erl	86.45% <ø> (ø)
src/jsone.erl	66.66% <100.00%> (+2.38%)	⬆️

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[sile]

Please let me know if there is someone who doesn't want the change of the map_unknown_value default value.
I'll merge this PR a week after if there is no such comment.