ENH: Make ExcelWriter more easily pluggable

[jtratner]

Basically, make a clear interface for ExcelWriter and allow others to plug into it. Unfortunately, it looks like we're stuck with using ExcelWriter directly as a class, so might take a bit more complexity to stick this in (i.e., a metaclass). If we can convert it to a function instead, can make a lot less magic. Check out this branch if you want to see what I'm talking about.

We'd end up with a separate config option for each extension (io.excel.xlsx.writer, io.excel.xls.writer, io.excel.xlsm.writer) (can't do the reverse because io.excel.writer.xls would match all the different filetypes).

Client code then just has to implement save and write_cells and define supported_extensions and register their their class with register_engine in pandas/io/excel, then it's easily available under to_excel as an engine parameter.