Add an option to skip additional files check #75

We use wp-cli in production on multiple very large sites to check for WordPress updates & watch for modified files via our monitoring system. We regularly run into an issue where wp core verify-checksums takes many minutes to run, due to the fact that the code searches the entire website root recursively to make a list of files for the "File should not exist" check.

I know that there is already an open issue to add the ability to skip files which would help with this issue, but I believe that an easier solution for very large sites like ours would simply be to add a command line switch that would disable the "File should not exist" check entirely.

In our environment, we use a modified version of wp-cli with lines 106 through 114 of Checksum_Core_Command.php commented out, which works perfectly and the verify-checksums process takes just seconds to complete.

Since adding the ability to skip files is probably a larger and more far-reaching task (which comes with it's own downsides - for example, we would need to maintain a list of the subdirectories to skip), would it be possible to just have a flag to disable the "additional files" feature at runtime to at least satisfy the simple use case of not wanting all files recursively listed?

Thanks and thanks again for the hard work on this tool!