Recognize individual files to process from CLI (#23)

Recent refactor of code introduced a regression: provide individual
files to CLI resulted in all *.cr files being processed instead, so
given file was ignored.

This change restores the original behavior that allows more fine
grained execution.

src/ameba/cli/cmd.cr

@@ -20,7 +20,7 @@ module Ameba::Cli
       parser.on("-v", "--version", "Print version") { print_version }
       parser.on("-h", "--help", "Show this help") { show_help parser }
       parser.on("-s", "--silent", "Disable output") { opts.silent = true }
-      parser.unknown_args { |f| files = f if f.any? }
+      parser.unknown_args { |f| opts.files = f if f.any? }
 
       parser.on("-c PATH", "--config PATH", "Specify configuration file") do |f|
         opts.config = f