added support for github enterprise

Feel free to merge this or not... but I find it useful for my given circumstances.
Others may find my version useful if they also desire this functionality.
Allows you to freely migrate repos to and from github enterprise and github.com.
--------------
I 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
```