choco pack - exclude files

`choco pack` tends to be hungry, grabbing everything in the current directory when it is run. It would be useful if there was an `--exclude` command line option similar to what NuGet itself offers. The use case I'm running into is when I need to use `choco pack` manually in a directory for a package normally managed by AU, to avoid picking up the update.ps1 script next to the nupkg file.

Start at the choco pack command entry point and compare the requested --exclude behavior with the NuGet behavior referenced in the issue. Use the manual package directory containing update.ps1 as the reproduction case; done means choco pack accepts --exclude and does not include matching files.

Written by the indexing model from the issue text.