* add an --exclude-file option to 'spack mirror create' which allows a user to specify a file of specs to exclude when creating a mirror. this is anticipated to be useful especially when using the '--all' option

* allow specifying number of versions when mirroring all packages

* when mirroring all specs within an environment, include dependencies of root specs

* add '--exclude-specs' option to allow user to specify that specs should be excluded on the command line

* add test for excluding specs