Add exclude flag to list and pack commands

[jankaszel]

Continuing work from #108 on adding an -exclude flag for ignoring particular directories to both the list and pack commands. Includes tests and documentation.

Tests fail at the moment. This is due to bugs within the testing framework as I've laid out in #108 (comment):

Tests fail though, as they do on the main branch atm. @markbates to me it occurs that there is a bug in pkging/pkgtest that will somehow keep references to the first directory created by pkgtest.NewRef() for subsequent tests. As the first directory will be deleted after the first test passed, subsequent tests will fail due to the missing directory. Didn't find a solution for that bug, but if you comment out line 26 in the above parser_test.go, tests for the excludes argument will pass.

[jankaszel]

@markbates cheers, any plans to get this merged?

[aeneasr]

Would be great if this could be merged!

[cornelk]

@markbates any chance to get this merged?

[jankaszel]

FYI the embed library introduced in Go 1.16 could offer a viable alternative for folks running into issues with pkger related to large directories that should be ignored: https://golang.org/doc/go1.16#library-embed