RFE: spec parse mode for preamble only

[pmatilai]

There are various use-cases where you're only interested in some key preamble values, typically surrounding name, version etc. Parsing the entire spec in these cases is just rendundant, and subject to more version incompatibilities than just the preamble area.

Such a mode could, at least optionally, ignore any unknown tags to allow at least the basic bits to be queried across maximum range of rpm versions.