Fix keyval funtion: pandoc did not parse options in braces correctly.…

[schrieveslaach]

… Additionally, dot, dash, and colon were no valid characters. Therefore, options, for example, of the lstlsiting won't be parsed correctly. See following LaTeX code

\begin{lstlisting}[language=Java, caption={Java Example}, label=lst:Hello-World]
public class World {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}
\end{lstlisting}

The resulting CodeBlock did not contain label and caption as attributes.

This PR fixes this issue.

[jgm]

More idiomatic:

  val <- option "" $ char '=' >> braced <|> (many1 (alphaNum <|> oneOf ".:-|\\"))

[schrieveslaach]

done

[jgm]

Thanks!