Add directory argument to mdbook test

Author: mattico

src/cmd/test.rs

@@ -7,7 +7,10 @@ use mdbook::MDBook;
 pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
     SubCommand::with_name("test")
         .about("Test that code samples compile")
-        .arg_from_usage("-L, --library-path [DIR]... 'directory to add to crate search path'")
+        .arg_from_usage("-L, --library-path [DIR]... 'directories to add to crate search path'")
+        .arg_from_usage(
+            "[dir] 'A directory for your book{n}(Defaults to Current Directory when omitted)'",
+        )
 }
 
 // test command implementation