feat(starter): allow on-prem github instances

Pull request checklist

Please check if your PR fulfills the following requirements:

[x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
[x] Build (npm run build) was run locally and any changes were pushed
[x] Tests (npm test) were run locally and passed
[x] Prettier (npm run prettier) was run locally and passed

Pull request type

Please check the type of change your PR introduces: - [ ] Bugfix - [x] Feature - [ ] Refactoring (no functional changes, no api changes) - [ ] Build related changes - [ ] Documentation content changes - [ ] Other (please describe): ## What is the current behavior?

Users are unable to pull stater templates from on-prem instnaces of github

GitHub Issue Number: Supersedes https://github.com/ionic-team/create-stencil/pull/45 - thanks to @gfellerph for their work on this!

What is the new behavior?

add support for self hosted github repositories. when generating the url to download a starter from, check for the appropriate environment variables first.

there are two sources of configuration that are accepted - one at invocation time (the command line) and one in .npmrc. see the README changes in this commit for additional details.

Does this introduce a breaking change?

[ ] Yes
[x] No

Testing

In addition to new unit tests, this can be manually tested

Build the project - from the project root, npm ci && npm run build
Link the project (so we can invoke it with npm init later) - from the project root, npm link
Create a new directory somewhere where we don't care too much what happens to it, mkdir /tmp/on-prem-test
Copy index.js from the root of the project to your new directory
First, let's test command line invocation: stencil_self_hosted_url=https://your_self_hosted_github_repo.com node index.js component foo-bar. You should see an error like:
```
code: 'ENOTFOUND',
syscall: 'getaddrinfo',
hostname: 'your_self_hosted_github_repo.com'
}
```
Next, let's test the .npmrc - delete index.js locally
npm link create-stencil from your tmp dir
Create an .npmrc: echo stencil_self_hosted_url=https://github. > .npmrc
Run npm init stencil. Expect an error message like that in step 5
Other information

ionic-team / create-stencil