expose hid_version_str

`hid_version_str` was added to the underlying C library in 0.10.0,
in libusb/hidapi#192

Note that the method added here returns the version of the C library,
which is different from the version of cython-hidapi, i.e. the `hid` python package.
It would be good to also make the latter available at runtime somehow.

chid.pxd

@@ -37,3 +37,4 @@ cdef extern from "hidapi.h":
     int hid_get_serial_number_string(hid_device*, wchar_t *, size_t)
     int hid_get_indexed_string(hid_device*, int, wchar_t *, size_t)
     wchar_t *hid_error(hid_device *)
+    char* hid_version_str()

hid.pyx

@@ -70,6 +70,14 @@ def enumerate(int vendor_id=0, int product_id=0):
     hid_free_enumeration(info)
     return res
 
+def version_str():
+    """Return a runtime version string of the hidapi C library.
+
+    :return: version string of library
+    :rtype: str
+    """
+    return (<bytes>hid_version_str()).decode('ascii')
+
 cdef class _closer:
     """Wrap a hid_device *ptr and a provide a way to call hid_close() on it.