Allow custom file/directory locations

[dannyadair]

At the moment there are some naming conventions:

The YAML settings documents, pages, images and templates have to be called exactly that, so does variants.
So do special settings like compress_pdf and svg2pdf_backend.
That is all fine and we don't have to go meta on it.

Technically configurable but in my opinion also ok to prescribe:

• style and theme have to be called exactly that
• highlight_color has to be called exactly that (and the tag it replaces has to be <highlight>)
• bake.py has to be called exactly that, and has to define a process_document() function taking a PDFBakerDocument

However:

• documents are just names, and for each document a directory has to exist by that name
• config.yml must be the filename of the document configuration - it must exist in that directory
• pages are just names, and for each page there has to be a .yml file inside pages/
• images are file names, and for each image a corresponding file has to be inside images/
• templates are file names, and for each template a corresponding file has to be inside images/
• the build directory is always build on the same level as the main config file
• the dist directory is always dist on the same level as the main config file

(I may have overlooked other naming conventions.)

That's a little bit restrictive and black magic - some of these files might already exist in other locations and shouldn't have to move.

Therefore:

• For every document, allow to split so it has a "name", "file" and/or "config_file" - the latter two being relative or absolute paths (but "relative" to the main config file).
• For every page, allow to split up so it has a "name" and a "file" (again relative/absolute path to the config file)
• For every image, allow to split up - that's already possible for providing "type" - so it also has a "file" (again relative/absolute path to the image file)
• For every template, allow to split up so it has a "name" and a "file" (again relative/absolute path to the template file)
• Allow the configuration of the build directory in the main configuration file (name or path)
• Allow the configuration of the dist directory in the main configuration file (name or path)

[dannyadair]

Implemented in version 0.5.0

• string: absolute path or path relative to the config file (name is stem)
• path/name: absolute path or path relative to the config file, name

All settings can be overridden on all levels (except a page always uses build/dist directory of its document)