support for parameterized github api url

[furlongm]

fixes #55

[jcfr]

Thanks for the contribution 👍

Would it be possible to have a look at the following error:

./github_release.py:188: [E999] SyntaxError: invalid syntax
    global _github_api_url = github_api_url
                           ^

I suggest to do this instead:

global _github_api_url
_github_api_url = github_api_url

[furlongm]

Apologies, fixed that error. Travis checks seem to be failing now due to TOKEN not existing. Not sure if my change introduced that error?

[jcfr]

Thanks for updating the PR.

Not sure if my change introduced that error

The failure are independent of your changes, the issue is that the GITHUB_TOKEN is not available for PR submitted from fork. I will fix the test so that they are skipped

[jcfr]

As soon as the CI is done, I will release a new version of the package on PyPi.

[jcfr]

See https://travis-ci.org/j0057/github-release/builds/518909935