Add GitHub Enterprise support

[sicot-f]

I needed to crawl an internal instance of GitHub Enterprise (GHE).
I added an environment variable GH_ENTERPRISE_URL to give the URL of the GHE instance. If left empty, it will crawl the public GitHub.
Close #19
Hope you find that useful.

[zkoppert]

Thanks for this @sicot-f! I will run the tests and take a look.

[sicot-f]

I added a default value for GH_ENTERPRISE_URL as mypy pointed out

[zkoppert]

Looks great! Errors are related to a file you didn't touch here so I'll merge. Thanks again!! ✨

[ChrisMeeusen]

what is the format for the GH_ENTERPRISE url? We are on GHE but using this format doesn't seem to work

https://github.com/enterprises/MY_ENTERPRISE_NAME_HERE

[sicot-f]

what is the format for the GH_ENTERPRISE url? We are on GHE but using this format doesn't seem to work

https://github.com/enterprises/MY_ENTERPRISE_NAME_HERE

In my case, we have a GH enterprise instance deployed internally so the value of GH_ENTERPRISE_URL is the intranet URL to reach GH. I was not aware there was GH enterprise on github.com domain.

As a workaround maybe you can try to o use the regular ORGANIZATION variable with value enterprises/MY_ENTERPRISE_NAME_HERE leaving GH_ENTERPRISE_URL blank, eg.:

GH_TOKEN='xyz'
TOPIC='inner-source'
ORGANIZATION='enterprises/MY_ENTERPRISE_NAME_HERE'
GH_ENTERPRISE_URL=' '