Set up build.sbt to auto-import morph code in REPL

To make testing in the REPL easier, configured SBT to auto-import all
morph classes, objects, and implicit conversions into the current scope
when the REPL is started. That includes the following imports:

* morph.ast._
* morph.ast.Implicits
* morph.ast.DSL._
* morph.parser._
* morph.parser.Interface._
Updated sbt config with instructions on build dep