Kidiff

Added
- Support for user defined layer names.

Fixed
- The 'only in XXX' message worked only the first time, after caching the
files it wasn't informed anymore.

Changed
- Now PCB layers are cached by layer ID, not name.
- Lines in ex/include lists that begin with are just ignored.
Before we let them fail to match because the names weren't valid.
This avoids confusing warnings.

Added
- Support for adding/removing SCH sheets

Fixed
- KiCad 5 problems with inner layers (undefined names)

Added
- Support for adding/removing PCB layers
- Option to specify the layers to use, instead of excluded

Changed
- The red/green colors to match text mode diff tools

Added
- Multi-page schematic support.
- More control for the output name and generated files.
- Option to just populate the cache.
- Support for layer numbers in the exclude file.

Changed
- Error codes for old/new file invalid.
The old ones are the reserved by Python.

Added
- Stats diff mode
- Alternative support for Ghostscript