Make datadoc windows compatible

[francescalb]

I get an issue with reading the context file as it does not convert it correctly to windows path.

running datadoc add xxx --dump=kb.ttl ends with this error message:

File "C:\xxxx\tripper\datadoc\dataset.py", line 362, in get_jsonld_context
with open(CONTEXT_PATH[7:], "r", encoding="utf-8") as f:
~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 22] Invalid argument: '/C:/xxxx/tripper/context/0.2/context.json'

As you see the slash/backslash seems to be wrong.

[francescalb]

Closed with #303