Plugin infrastructure in corpus2 readers: plugin loading code in pwrnlp/plugin, chanegs all around to accomodate

Revert "add has_more to readers". Not needed with the upgraded shared_ptr swig wrappers (and also not fully correct).

This reverts commit 99388c53.

* make the factory always return writers via shared_ptr
* add a PathWriter wrapper for opening a stream for the lifetime of a normal Writer
* wrap writer factory exceptions
* hide some writer factory implementation details in dteail:: sub namespace
* rename TokenWriter::create to create_stream_writer, add create_path_writer
* bump library version

Allow custom "ign" tag equivalent in tagsets, fix tests accordingly as now a valid ign tag is required