Add option to underscore field/table names

[gene-pavlovsky]

Currently it's possible to give custom names to tables. However, it's a bit tedious compared to the automatic naming. I didn't see a possibility to give custom names to fields.

Personally I prefer the tables and fields to be all-lowercase with underscores to separate words.
It would be nice to have a compile-time flag that would select the style of table/field name generation.
The current style would then be the default one to maintain backwards compatibility with existing code. Other styles like the one I proposed would be possible to select when compiling, e.g. -Dtink_sql_naming=lowercase_underscore

This would remove the need to manually redefine table names
@:table("post_tags") var postTags:PostTags;
And it would also apply to field names, e.g. firstName field in haxe would map to first_name field in the database.