Add ability to manually declare reachbility in DCE
Add CLI option support to Scala.js partest interface

The following options are now supported (and replace the corresponding
environment properties). Note that they are mutually exclusive.

    --blacklisted      Run blacklisted tests only
    --buglisted        Run buglisted tests only
    --whitelisted      Run whitelisted tests only
    --unknown          Run unknown tests only
    <name> ...         Run exactly these tests

A couple of usage examples

    sbt> scalajs-partest-suite/testOnly -- --unknown
    sbt> scalajs-partest-suite/testOnly -- run/amp.scala run/t1466.scala
Add MemVirtualWriters
Remove dependency on lift-json. Introduce JSON helpers
Fix #456: Run partest with fastOpt and Node.js

This adds the following options to the partest runner:

    --noOpt      Don't optimize, use Rhino (default)
    --fastOpt    Run DCE and run with Node.js

These options can be combined with the test selectors. Example usages:

    sbt> scalajs-partest-suite/testOnly -- --fastOpt
    sbt> scalajs-partest-suite/testOnly -- --fastOpt --unknown
    sbt> scalajs-partest-suite/testOnly -- --fastOpt run/amp.scala run/t1466.scala