Make sure the shell script is executable:
$ chmod +x /path/to/github.sh
In your ~/.bash_profile
add an alias for the script:
alias github='~/path/to/github.sh'
Restart your terminal.
To open the GitHub page for your repo:
$ github
To open a specific GitHub subpage for your repo, just add the identifier (url) of the page. For Pull Requests, it would look like this:
$ github pulls
For the wiki:
$ github wiki