* Add custom error variant
This can be useful to propagate errors not conforming to available
variants. Also to convert other errors in `crate::Error` without having
explicit conversion defined, with `error.to_string()`
* Implement `Tag` and define protocol serialization
A Tag structure have been implemented with dedicated field types. Then
custom serde serialization is derived to map the structure to current
protocol json array as per NIP01.
This adds compile and run time type checking to always ensure wrong
string data are never stored or processed. With strict typed fields and
custom serde derivation this checks can be done at time of serialization,
saving work for internal handling of the actual data.
tests for possible data violations are added, and gives good example of
kind of errors it will through for different cases.
* Use String for URL
