Use version specified in .node-version if exists

It's pretty common to have a file `.node-version` in a repo to specify
the version of node to use.

This will pick up the version specified in this file and use that as
the default when `Config` is initialised.

nodeenv.py

@@ -106,6 +106,7 @@ def _load(cls, configfiles, verbose=False):
         """
         Load configuration from the given files in reverse order,
         if they exist and have a [nodeenv] section.
+        Additionally, load version from .node-version if file exists.
         """
         for configfile in reversed(configfiles):
             configfile = os.path.expanduser(configfile)
@@ -133,6 +134,10 @@ def _load(cls, configfiles, verbose=False):
                         os.path.basename(configfile), attr, val))
                 setattr(cls, attr, val)
 
+        if os.path.exists(".node-version"):
+            with open(".node-version", "r") as v_file:
+                setattr(cls, "node", v_file.readlines(1)[0].strip())
+
     @classmethod
     def _dump(cls):
         """