Validate/pre-process data from nodes

[stsewd]

Some nodes require some type of data that we need to validate, like TestBlock.validator (this should be an importable module).
Or CodeBlock where the content should be a list of lines. Or Admonition.type where it should be one of the three types. Maybe we can do the validation like django rest framework does, or something more simple like the rst directive does

lira/lira/parsers/rst.py

Lines 73 to 76 in 298679e

	option_spec = {
	"help": str,
	"validator": importable,
	}