Add set "features" to Context class.

yangson/context.py

+from typing import Dict, MutableSet
 from .constants import YangsonException
 from .statement import Statement
 from .typealiases import *
@@ -20,6 +21,8 @@ class Context:
     ns_map = {} # type: Dict[YangIdentifier, YangIdentifier]
     """Map of module and submodule names to namespaces."""
 
+    features = set() # type: MutableSet[QName]
+
     @classmethod
     def resolve_qname(cls, mid: ModuleId,
                       qname: QName) -> Tuple[ModuleId, YangIdentifier]:

yangson/datamodel.py

@@ -31,6 +31,9 @@ class DataModel:
             for item in yl["ietf-yang-library:modules-state"]["module"]:
                 name = item["name"]
                 Context.ns_map[name] = name
+                if "feature" in item:
+                    Context.features.update(
+                        [ name + ":" + f for f in item["feature"] ])
                 rev = item["revision"] if item["revision"] else None
                 mid = (name, rev)
                 ct = item["conformance-type"]