contextualize CEA installer when using --yarn flag

Type of Change

• [ ] New Feature Request
• [ ] Documentation
• [x] Improvement / Suggestion
• [ ] Bug
• [ ] Other (please clarify below)

Summary

When running

$ npx create-evergreen-app my-app --yarn

The user will see this

Installing project dependencies...
yarn install v1.12.3
warning package-lock.json found. Your project contains lock files generated by tools other than Yarn. It is advised not to mix package managers in order to avoid resolution inconsistencies caused by unsynchronized lock files. To clear this warning, remove package-lock.json.

And also

 npx create-evergeen-app my-app --yarn
.
.
.
And then launch your project with: npm start
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Details

If the --yarn flag is passed, we should delete package-lock.json before running install and make sure the instructional text references Yarn instead of npm.

related to #23

Also, we should update the success message that is output too

$ npx create-evergeen-app my-app --yarn
.
.
.
And then launch your project with: npm start
-------------------------------------------------------

We should have it say yarn start