update several functions for submode awareness, and add `sbt-paste-region`

- add: comint buffer now has the buffer-local variable, `sbt:submode` holds the current _submode_ of the sbt buffer.
- update: `sbt-command` will return to the _sbt_ submode before executing a sbt command.
- update: `sbt-send-region` will enter console mode(Scala REPL) if sbt buffer is in _sbt_ submode.
- add: `sbt-paste-region` behaves like `sbt-send-region`, except that it uses paste mode of Scala REPL.
add `run-scala` command for easy launching of Scala REPL.

In addition,

- update `README.md` to introduce `sbt-paste-region` command
- preliminary support for _no-exit_ argument for `sbt-paste-region`