Avoid problems with offline usage

[ehoogerbeets]

• When using opencc-js while not connected to a network, the fetch of the txt data fails. Depending on opencc-data for the node implementation avoids that problem
• If your machine doesn't have the right certificate authorities, then using fetch to get things on cdn.jsdelivr.net will fail with an exception. Using local data files avoids that problem.
• Loading local files is faster than downloading over the internet

[sgalal]

Published a new version v0.2.2

[ehoogerbeets]

Thanks @sgalal for the quick review and publish!