Add scoop prefix command

[tresf]

Sometimes, the path to an app's root directory is needed, such as locating a library-only-app that's not exposed by scoop which.

Usage:

scoop prefix <app>

Example:

scoop prefix libsndfile

C:\Users\Administrator\scoop\apps\libsndfile\current

Documentation:

Usage: scoop <command> [<args>]

Some useful commands are:

  alias       Manage scoop aliases
  [...]
+ prefix      Returns the path to the specified app