Improvements to bpkg init

- Generate README.md file with description and install directions pre-filled
- Generate $NAME.sh file with boilerplate, including source-check, usage,
-   argument processing, and version print.
- Nice default for package.json "install" field
- Initialize git repo if one does not exist
- Fix issue with prompt where delete key erased prompt, by using `read -p`
Merge pull request #27 from ccarpita/new/init-source

Improvements to bpkg init