Add label to load function

[crdoconnor]

Feature requested in gjcarneiro/yacron#4

Example:

try:
    doc = strictyaml.load(data, CONFIG_SCHEMA, label=path).data
except YAMLValidationError as ex:
    print(ex)

strictyaml.exceptions.YAMLValidationError: while parsing a mapping
unexpected key not in schema 'killTimeoutx'
  in "/path/to/whatever.yaml", line 73, column 1:
      killTimeoutx: '0.5'
    ^ (line: 73)

[gjcarneiro]

+1