cpkg rewrite usage

cpkg-usage

$ cpkg

cpkg <command> <...>

Commands:
  list [pkg]                             List packages or versions of [pkg]
  use [global|(session)] <pkg>-<ver>     Use <pkg>-<ver>
  drop [global|(session)] <pkg>-<ver>    Stop using <pkg>-<ver>

In progress:
  search <pkg>                           Search build files and packages
  install [binary|(build)] <pkg>         Install <pkg>
  remove <pkg>                           Remove <pkg>

$ cpkg list
Listing packages:
[ ] python-2.7.2
[ ] python-2.7.3
[ ] ruby-1.9.3p125
[ ] ruby-1.9.3p194
[ ] yaml-0.1.4
$ cpkg use python-2.7.3
Adding FreeBSD--amd64--python-2.7.3 to the session profile.
$ cpkg list
Listing packages:
[ ] python-2.7.2
[S] python-2.7.3
[ ] ruby-1.9.3p125
[ ] ruby-1.9.3p194
[ ] yaml-0.1.4
$ cpkg use global yaml-0.1.4
Adding FreeBSD--amd64--yaml-0.1.4 to the global profile.
$ cpkg use global ruby-1.9.3p194
Adding FreeBSD--amd64--ruby-1.9.3p194 to the global profile.
$ cpkg list
Listing packages:
[ ] python-2.7.2
[S] python-2.7.3
[ ] ruby-1.9.3p125
[G] ruby-1.9.3p194
[G] yaml-0.1.4
$ cpkg drop global yaml-0.1.4 
Dropping FreeBSD--amd64--yaml-0.1.4 from the global profile.
$ cpkg drop global ruby-1.9.3p194
Dropping FreeBSD--amd64--ruby-1.9.3p194 from the global profile.
$ cpkg drop session python-2.7.3
Dropping FreeBSD--amd64--python-2.7.3 from the session profile.
$ cpkg list
Listing packages:
[ ] python-2.7.2
[ ] python-2.7.3
[ ] ruby-1.9.3p125
[ ] ruby-1.9.3p194
[ ] yaml-0.1.4
$