Allow obtaining global symbols (like modules, their functions, attributes) even without created YaraFile

[metthal]

The motivation here is to be able to obtain list of all functions in certain module without creating YaraFile object. We should be able to do something like yaramod::getSymbol("cuckoo") and it would search only for global symbols. Alternatively, you would also be able to provide Import Features in there if needed. This would be useful for listing those symbols (for example for code suggestion purposes) and not having to create dummy YaraFile object with all imports.

[TadeasKucera]

In #143 we introduced dynamic modules from JSON and also a method Yaramod::modules that enables access to all symbols from used YARA modules.