Add an exclude key to the manifest

Add an exclude key to the manifest

This key is used to help determine the set of input files for a package. The
normal method (via walking or git ls-files) is filtered via all the patterns
provided in `exclude` of the project section.

The toml key is a string array corresponding to a set of glob patterns according
to the syntax of libglob (provided in the standard distribution). The set of
files normally found will be filtered by each pattern, and if any pattern
matches then the path is excluded.

This will later on be used for `cargo package` when generating a tarball to get
uploaded.