Allow private repository URLs when generating README #134

When the README is generated, each command has a very convenient link to its own source code. However link is only generated if the whole project has a public GitHub repository URL (github.com/...).

I am using my company's enterprise GitHub which has a different URL (github.mycompany.com/...). So, no links are generated in my README. This check is happening on this line of code.

I would like oclif to support any repository URL to handle the case of privately hosted, enterprise version control.