Simplify CLI semantics

[booniepepper]

• dt should always evaluate its args, print the final value(s), and then exit successfully
  • dt 1 1 + should print 2 and exit (no getlines at all)
  • echo 2 | dt 1 + should print 3
  • dt 1 1 + | cat should print 4
  • dt repl will launch a repl (or dt --interactive or dt -i or something)
• consider always streaming input by default (i.e. the args are evaluated over each line) and do the 1.x thing of reading all lines first as a command that reads until EOF