Pretty "show"-command

Tried `show` and found the output quite unreadable.

    $ ./bin/composer show --installed
    installed:
      justinrainbow/json-schema 1.1.0              
      seld/jsonlint             1.0.0              
      symfony/console           dev-master d214b14 Symfony Console Component
      symfony/finder            v2.1.2             Symfony Finder Component
      symfony/process           dev-master f029d06 Symfony Process Component

Suggestions are welcome :) I decided to remove the formatting characters (`[<version>]` and `:` to save some space. Especially with many entries the different parts are easy distinguishable