Misc updates

- Renamed global variable to GitHubCred to minimize risk of interfering
with existing global credential object
- Updated Get-GistContent with a new parameter to send the Gist to a new
ISE tab
- Updated Send-Gist so that if text is selected, only the selected text
will be created as a new Gist. Otherwise the whole file will be used
- Updated README.md
Merge pull request #1 from gravejester/master

* Renamed global variable to GitHubCred to minimize risk of interfering with existing global credential object
* Updated Get-GistContent with a new parameter to send the Gist to a new ISE tab
* Updated Send-Gist so that if text is selected, only the selected text will be created as a new Gist. Otherwise the whole file will be used