add feature to import [e]bnf grammars

[d0c-s4vage]

gramfuzz should have a method analogous to the load_grammar method that will parse a grammar file and create analogous gramfuzz structures in memory. Maybe something like:

fuzzer.import_grammar("path/to/test_grammar.bnf", type="bnf")

Both ebnf and bnf grammars should be allowed.

aside it may be cool (not sure how useful really though) to export grammar rules from memory into a file as well.

[d0c-s4vage]

This would be fantastic - bumping this one up