Added the new LexerResult to use as an Error.

The code now returns a result that has the TokenStream. For now
it is returning any Error, but later there should be a lexical analysis
error type.

examples/basic.rs

@@ -46,7 +46,7 @@ impl std::fmt::Display for MarkdownTokenType
 
 // Define how you want to interpret base tokens
 pub fn transform(input: &TokenStream<TokenType>)
-                 -> TokenStream<MarkdownTokenType>
+                 -> Result<TokenStream<MarkdownTokenType>, Box<dyn std::error::Error + Send + Sync>>
 {
    let mut output = TokenStream::new();
 
@@ -157,11 +157,11 @@ pub fn transform(input: &TokenStream<TokenType>)
       i += 1;
    }
 
-   output
+   Ok(output)
 }
 
 
-fn main() -> Result<(), Box<dyn std::error::Error>>
+fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>>
 {
    let mut path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
    path.push("examples/example.md");