Suggest: Add a env variable to let user select url to download dict.

[A1A2B3X]

In default, pyopenjtalk will download dict from github

But github may very slow in some region, so I suggest use a env variable to let user select download url, and they can get much faster downloading speed.

[r9y9]

#36 will eliminate downloading from github and may resolve your issue. I'd be happy to accept a PR in anyway.