Allow scripts to automatically eject (See #346 instead)

[Zomis]

Following the rejection of PR #202 , fixes #196.

In a comment on #202 it was suggested to refactor eject.js to make it possible for other scripts to do the eject for you.

Using the changes in these two commits, I was able to make a script like this from a CRNA-project:

const rns = require("react-native-scripts/build");
rns.eject({
  ejectMethod: "raw",
  enteredName: "ProjectName",
  enteredDisplayname: "My project"
});

I didn't manage to make a index.js file go to react-native-scripts/index.js as taskfile.js specifies to build src directory, if anyone else has another solution for that (or anything else that can be improved in this PR) then feel free to improve on that or suggest improvements.

[Zomis]

Sorry for mixing up the issue id in one of the commits and in my branch name, I just noticed this mistake now :(

[Zomis]

Closing this one because of the commit message mistake and the branch name mistake. See #346 instead