2.3.0 Release

Add `lein-generate` task and modify `lein` task:

- The `lein-generate` task generates a `project.clj` file for the
  project based on the boot environment. See `boot [help lein-generate]`
  for all the details.

- The `lein` task now overwrites an existing `project.clj` file and does
  not delete the generated one on exit. See `boot [help lein]` for all
  the details here, too.