SilFrontend API for Viper frontend usage

[marcoeilers]

• Offers a new trait with two methods for easy usage of SilFrontend by actual Viper frontends. Compared to working with Verifier instances directly, these take care of plugins (i.e., they run the relevant plugin phases automatically; command line arguments can be used to select which plugins to run) and don't require calling a bunch of boilerplate methods. Compared to existing SilFrontend classes, these are made to be used by frontends (without parsing etc.) and correctly pass command line options to plugins.
• Usage is simple:

  val f = new SiliconFrontendAPI(reporter)  // or CarbonFrontendAPI, or MinimalSiliconFrontendAPI (skips consistency checks)
  f.initialize(args) // command line arguments without any dummy filename
  val res1 = f.verify(program1)
  val res2 = f.verify(program2)
  f.stop()
  

• Re-added option to disable default plugins.

Tested with adapted versions of Prusti and Nagini.

[vakaras]

LGTM. Thanks!