Add `jsone:ip_address_to_json_string/1` function

[sile]

This PR adds a utility function jsone:io_address_to_json_string/1 to encode IP addresses (i.e., inet:ip_addresss() values). This function can be specified as the value of the map_unknown_value encoding option. Please see the doc of the function and the following example for the detail.

> EncodeOpt = [{map_unknown_value, fun jsone:ip_address_to_json_string/1}].

> jsone:encode(#{ip => {127, 0, 0, 1}}, EncodeOpt).
<<"{\"ip\":\"127.0.0.1\"}">>

> {ok, Addr} = inet:parse_address("2001:DB8:0000:0000:0001:0000:0000:0001").
> jsone:encode(Addr, EncodeOpt).
<<"\"2001:db8::1:0:0:1\"">>

> jsone:encode([foo, {0, 0, 0, 0, 0, 16#FFFF, 16#7F00, 16#0001}], EncodeOpt).
<<"[\"foo\",\"::ffff:127.0.0.1\"]">>

Related issue: #49

[codecov-commenter]

Codecov Report

Merging #67 (8176612) into master (6ba5456) will increase coverage by 0.89%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   90.54%   91.43%   +0.89%     
==========================================
  Files           3        4       +1     
  Lines         296      327      +31     
==========================================
+ Hits          268      299      +31     
  Misses         28       28              

Impacted Files	Coverage Δ
src/jsone.erl	68.75% <100.00%> (+2.08%)	:arrow_up:
src/jsone_inet.erl	100.00% <100.00%> (ø)

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