Added JSONName attribute to specify a custom name to use when importing and exporting

[TJHeuvel]

Whenever a field has the JSONName property it will use that name, instead of the fieldname. This allows you to slightly deviate from the incoming or outgoing JSON.

[lbv]

Hi,

Thank you for the contribution. This seems interesting and probably something I'd like to add. A few comments:

• Regarding the JsonIgnore commit, it seems like it's the same issue from pull request #7. I'd invite you to share your point of view there, and leave this pull request only for the JsonName issue (please remove the commit https://github.com/TJHeuvel/litjson/commit/f503075789c06114bbcd7131aa36bda037c6facb if that's the case).
• Related to the previous point, I'd like to review the changes, but as it stands right now it's a mess. I know the original coding style of this library is terrible (I was young, more naive, and just as ignorant as today, which is to say a lot), but "fixing" things like whitespaces along with real code changes is not a very good practice if you want those changes merged into the original codebase.
• Before merging this type of changes (adding new attributes and the like), I'd like to check other projects first and see if they have similar features, mainly because I don't want to use different names for the same functionality. If other JSON libraries implement something similar, and they use the JsonName attribute, that's great; otherwise I'd like to choose the name for that symbol carefully based on what introduces the least disruption in codebases that currently use other libraries. If you can help me with this task of reviewing what other .Net libraries do, I'd appreciate it very much :).

Thanks.