If parse_literal is not present try fallbacking to parse_value(ast.value)

[rafalp]

Following idea was brought up in discussion for #24:

Maybe we could default to calling parse_value with ast.value when only one function is provided?

This requires further study. IntValue, StringValue and friends are obvious to deal with, but but complex types like ListValue may require some extra unpacking magic.

Still, if it is possible to pull off, it could be an excellent convenience for developers creating custom scalars, saving the need for potentially maintaining two very simiiar implementations, one doing isinstance(value, basestr) and other isinstance(value, StringValue).

[rafalp]

Another idea: for simplicity reasons we could only unwrap basic types, and still error on anything else, forcing the dev to handle more complex types (if they wish to do so).