Add a debug parameter

[JessicaTegner]

It could be useful to have a debug parameter to add more logging such as which files was found in the glob patterns in the convert* functions, what the command we are passing to pandoc look like and so on.

What do people think.

[saumzzz]

Along with that can we disable setting the log level for the root logger by default here?
https://github.com/JessicaTegner/pypandoc/blob/master/pypandoc/handler.py#L15

[JessicaTegner]

@saumcor indeed. We should make a pypandoc sublogger