Add metadata information to cached assets

When saving/reading user-provided syntaxes or themes, `bat` will now maintain a
`metadata.yaml` file which includes information about the `bat` version which was
used to create the cached files. When loading cached files, we now print an error
if they have been created with an incompatible version

closes #882

src/errors.rs

@@ -7,18 +7,27 @@ error_chain! {
         SyntectError(::syntect::LoadingError);
         ParseIntError(::std::num::ParseIntError);
         GlobParsingError(::globset::Error);
+        SerdeYamlError(::serde_yaml::Error);
     }
 }
 
 pub fn default_error_handler(error: &Error) {
+    use ansi_term::Colour::Red;
+
     match error {
         Error(ErrorKind::Io(ref io_error), _)
             if io_error.kind() == ::std::io::ErrorKind::BrokenPipe =>
         {
             ::std::process::exit(0);
         }
+        Error(ErrorKind::SerdeYamlError(_), _) => {
+            eprintln!(
+                "{}: Error while parsing metadata.yaml file: {}",
+                Red.paint("[bat error]"),
+                error
+            );
+        }
         _ => {
-            use ansi_term::Colour::Red;
             eprintln!("{}: {}", Red.paint("[bat error]"), error);
         }
     };