Replaces with-external-gems with Perl version

Also switches strategy to use a separate Gemfile.
Adds --verbose to `with-external-gems`
Adds workaround for lack of trailing newline
screenshot: moves `dmenu` after Dropbox init
screenshot: defaults dir before `dmenu`
screenshot: adds existing files to list
screenshot: refactors into functions, mostly
screenshot: simple check for existing files
clip: -O or -n = -out without trimming newline