support for github enterprise

have made extensive changes to how this works which allows for the
migration of issues to and from github enterprise and github.com, etc.

to do so, I had to add more explicit config options for source and
target.

example:
```
# Rename to `config.ini`, and keep it in the same folder as
`gh-issues-import.py`
# All optional fields have been commented out.

[source]
server = github.com
repository = example/sample_repo
username = user1@example.org
password = naked_password

[target]
server = github.example.org
repository = example/sample_repo
username = user1
password = naked_password

[format] # These can be adjusted based on your group's region and
language.

# Unless an absolute path is given, the templates are relative to the
current working directory.
# Example (and default) templates can be found in the `templates`
folder.
#issue_template =        templates/issue.md
#pull_request_template = templates/pull_request.md
#comment_template =      templates/comment.md

# If unsure, just using '%c' will default to the locale’s appropriate
date and time representation.
#date = %A %b %d, %Y at %H:%M GMT # Sample: Friday Sep 13, 2013 at
22:58 GMT
```