changes to move to an opt-in workflow

Fix https://github.com/google/stagehand/issues/91, @sethladd.

- change to an opt-in workflow
- no exceptions or events are sent w/o an explicit user opt-in
- all page views are changed to `main` w/o an opt-in - i.e., just a version check is run
- changed so that the CLI exits quickly, even if the analytics I/O takes a while
- updated tests and docs for these changes
- fixed one issue when analyzing the generated samples on the bots
does this also send the OS and Dart version? It's OK, just curious.