Directly parse ABNF rules from IETF RFCs

[nradov]

The Fuzzer class can currently only parse ABNF rules from files that contain only those rules as per the ANTLR grammar. So users who want to test implementations of IETF RFCs first have to manually copy and paste the rules into a separate file.
It would improve usability if this library could take the URI for an RFC as an argument and automatically extract the ABNF rules out of the entire document. However this appears to be a difficult problem in pattern recognition; it's hard to tell whether a particular line contains an ABNF rule, narrative text, or part of the header / footer. The bap aex tool may offer a starting point, however it doesn't appear to produce correct output for some RFCs.