remove the -o option; update docs and tests

Fix https://github.com/google/stagehand/issues/106. This is a backwards compatible change; the `-o` option is still available, but is deprecated and hidden now.

- remove `-o`
- default to creating the app in the current directory
- fail if the directory is not ~empty
- give the user the ability to force generation anyway
- update docs
- maintain backwards compatibility (for tools)

@sethladd