Allow parsing of incomplete rules

[metthal]

It would be nice to parse rules also in some kind of IncompleteRules mode which would ignore that some symbols might be missing from symbol tables. This would allow us to for example autoformat rules even without any includes which refer to other rules that are not part of the autoformatted rule.

For example let's say I have

rule abc {
    condition:
        xyz
}

xyz is not defined here but it might come from some include later therefore we'll just parse it silently and ignore it.

[metthal]

This should also allow us to obtain token stream from the parsed YARA file whenever there's a syntax error. For example let's say I have

rule abc {

This is not valid YARA but I can obtain token stream and by looking at the stream in reverse, I can infer that the next token should be meta and advise it to the user.

This one is more critical than the original use-case in the original post.

[metthal]

Another possible feature of this should be to ignore includes of files it can't find.