Add nested type handling specification to file_name rule

[fredpi]

Assuming I have a file that extends MyType.MySubtype like this:

extension MyType.MySubtype {
    // ...
}

I may call this file MyType.MySubtype+Extension.swift without triggering the file_name rule. I may also specify a custom file name suffix, so that I could name it MyType.MySubtypeExtension.swift.

In contrast, I cannot call my file MyTypeMySubtypeExtension.swift without triggering a warning. As such a naming (without a second . character in the file naming) may be preferred by some, I propose to extend the file_name configurability to allow for custom nested type separators (with . remaining the default).