Use .boot extension for boot scripts; add :strap option

* Boot scripts must have the .boot filename extenstion. This makes it
  possible to call boot either via the shebang or like `boot arg arg`
  when there is a `build.boot` file in the current directory. A distinct
  filename extension makes sense for boot scripts also because they are
  not exactly Clojure files, or EDN, or anything else. They need to be
  evaluated inside of the boot runtime.

* The `:strap` option to boot searches Maven for the latest stable
  boot.core version and emits a minimal boot script containing it.