Add custom name-rewriter option

[myieye]

fixes #42

Adds a custom-mapping name-rewriter.

[roji]

Thanks, looks good! Just some very minor nits.

[mphelt]

Hi, I don't know if I may suggest anything at this point.
Have you considered storing Func<CultureInfo, INameRewriter> _createRewriter instead of Func<string, string>? _customMapper ?

With such field another extension method could be added:
public static DbContextOptionsBuilder<TContext> UseCustomNamingConvention<TContext>(this DbContextOptionsBuilder<TContext> optionsBuilder, Func<CultureInfo, INameRewriter> createRewriter)
that would allow to easily inherit from existing rewriters:
public class RemoveSuffixSnakeCaseNameRewriter : SnakeCaseNameRewriter { ... }.

CustomNameRewriter would then be instantiated in NamingConventionSetPlugin through such factory method and not directly.

[myieye]

@roji Finally got around to incorporating your feedback

@mphelt certainly worth considering, but I'm not willing to put any more time into this currently. Sorry.

[myieye]

@roji can this be merged?

[barba3]

@roji do you have any objection with merging this PR? - The design in my head looks exactly like @myieye's.

Also, @mphelt's suggestion makes sense to me too, but in my humble opinion, it can be in a different PR.