Add windows support

Windows expects executables to have a .exe file extension, and the lack of
it causes various problems trying to run the plugin.

This commit fixes the filename for protoc on Windows.  It doesn't fix the
other main problem with running the plugin on Windows, which is that
building protobuf requires you to be in a MinGW environment and gets a bit
involved.  The expectation is that on Windows you'll have to build
protobuf yourself, but once you've done that the plugin should work as
normal.