provide an ability to exclude files/directories from bundles

Other than specifically src, lib, bundles, and vcs.history, it would be very useful to omit specific files or sub-directories from a bundle.

My first thought was to include extra_flags="--exclude data-raw", but since bundle forces the use of the internal tar (vice the tar executable), the extra_flags= argument is ignored.

Some options:

• Allow the user to override tar=. There are incompatibilities between internal and command-line tar, so this might not be possible, but it would allow the use of extra_flags= if done correctly.

• Add bundle(..., exclude=) where exclude is either a vector of files/dirs to remove completely or patterns to match against all files. This would require changing from files=basename(project) to an actual vector of files/dirs to include, appropriately filtered.

Start by locating the bundle implementation, the internal tar path, and the code that builds files=basename(project). Compare the proposed tar override with an exclude argument for files, directories, or patterns; done means a bundle can omit the requested paths without changing the existing default contents.