Add more options to PrinterConfig

[note]

Ideas of additional options in PrinterConfig:

• add Boolean option for repairing namespaces (i.e. automatically defining used namespaces in case they're not yet defined)
• add option which defines how to treat multiple attributes for the same elements (namely <a attr="val1" attr="val2"></a>). Exemplary behaviors - ignore it and print all of them, flatten them by concatenating them separated by spaces, use the last value, use the first value)

[note]

Another option - let user decide how to print empty elements: <a></a> vs <a />.

BTW for now <a></a> is implicitly assumed.