Make 'unrecognized-format' tweakable

[nulltoken]

User story. 'unrecognized-format' rule has been added in https://github.com/stoplightio/spectral/pull/798

It's a great addition.

Unfortunately, it isn't processed like a standard rule. As such, its severity cannot be changed through a ruleset, nor can it be ignored with an except entry.

Describe the solution you'd like This great rule should deserve the first-class citizen treatment!