[Swift4] Expanding CodableHelper with added date and JSON customisations

Created by: pcoltau

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

• Improving performance of CodableHelper by moving the default DateFormatter to a field variable.
• Added jsonDecoder and jsonEncoder field variables, making the JSON transformations customisable.
• Removed prettyPrint as parameter from encode function and moved it to the default JSONEncoder instance, in order to allow customisation of the prettyPrint feature.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03)