Add show command

[theory]

This would allow one to look at the info from a change or tag, or the contents of a script. Similar to git cat-file. Some examples that would show the output from info of a plan object:

sqitch show change df6a24dc05f00328e6c1dc3dca79a4e9e1627dd6
sqitch show change foo
sqitch show @beta1

And these would show the contents of files:

sqitch show deploy users_table
sqitch show revert users_table
sqitch show test users_table

[theory]

Done in 8d66654.