Enable separate generation for sub-folders

[OfekShilon]

Large projects still can generate unfeasibly large index.html files, to the point of breaking browsers (and just making navigation unwieldy). If the build output is separated to different directories, it should be easy enough to enable automatic separation of processing to them. Say by a command line switch --separate-folders.

E.g., if the build outputs to ./obj', ./obj/component1, obj/component2 - mimic this folder hierarchy under the html output path, and generate 3 different index.html + 3 sets of source htmls.

Since the translation units likely share common headers this is likely to induce duplicity (the same header html would be rendered in 3 different libraries with different remark annotations), but good chance it would be often worth it.

[OfekShilon]

Implemented by the --split-top-folders switch