Defined build rules

The build rules are defined in the file build/rules.bzl. A quick list is here:

Rule	Description
`asymptote(name, srcs, deps)`	This build rule converts Asymptote source files into images that can be included in the book. This rule can take any `*.asy` file in `srcs` and can depend on any `asymptote` rule in `deps`.
`dot_png(name, srcs, deps)`	This build rule converts a Graphviz source files into PNG images that can be included in the book. This rule can take any `*.dot` file in `srcs` and can depend on any rule in `deps`. The `.dot` file is laid out using the graphviz program `dot`.
`drawtiming_png(name, srcs, deps, output, args)`	Typeset a timing diagram using drawtiming.
`markdown_lib(name, srcs, deps)`	This build rule makes a library out of `*/md` files. `deps` may be any `markdown_lib` or `asymptote` or other such rule, and those will be used correctly.
`ebook_epub(name, deps, metadata_xml, title_yaml, args)`	This build rule assembles all `markdown_lib` rules in sequece and produces a book named `[name].epub`
`ebook_kindle(name, deps, metadata_xmp, title_yaml, args)`	This build rule assembles all `markdown_lib` rules in sequence and produces a book named `[name].mobi`
`ebook_pdf(name, deps, metadata_xmp, title_yaml, args)`	This build rule assembles all `markdown_lib` rules in sequence and produces a book named `[name].pdf`
`neato_png(name, srcs, deps)`	This build rule converts a Graphviz source files into PNG images that can be included in the book. This rule can take any `*.dot` file in `srcs` and can depend on any rule in `deps`. The `.dot` file is laid out using the graphviz program `neato`.
`plantuml_png(name, srcs, deps)`	This build rule converts a PlantUML source files into PNG images that can be included in the book. This rule can take any PlantUML-formatted `*.txt` file in `srcs` and can depend on any rule in `deps`.
`pandoc_standalone_html(name, deps, metadata, toc, title, args, filters)`	Use pandoc to convert the `markdown_lib` deps listed into a standalone HTML file.
`pandoc_chunked_html(name, deps, metadata, toc, title, args, filters)`	Use pandoc to convert the `markdown_lib` deps listed into a set of chaptered HTML resources. This is probably the best way to generate a set of self-contained files.

Common parameters

args: (list[string]): verbatim arguments to be passed to the underlying program.
deps: (list[Label]): dependency labels, can be any generated targets.
filters: (list[Label]): a list of pandoc filters to apply, in the order that they need to appear in the pandoc command line.
toc: (bool): whether to generate a table of contents.
metadata: (Label): A label representing a YAML metadata file. Note that quite a few of these may be specified as preamble to regular pandoc markdown.