Allow to load custom readers with new dict setting: READERS.

This adds a new dict setting, READERS, containing extensions -> Reader class, allowing to load custom readers. I do not know if there was another way to load readers.

My use case is that I need another RstReader, that do not rely on html4css1 (mostly because I do not like the output, so not html5). so I created a mypelican.RstReader and loaded it this way:

    import mypelican.RstReader
    READERS = {'rst': mypelican.RstReader,}