Add align column attr

[aks]

First, thanks for a nice table formatter.

Second, I've added some column "helper" functions to make the tp argument hashes easier to build. These are the in column_helpers.rb file. There are some changes in the other files to support the new :align attribute.

I've also improved the formatters to support right, and center alignment for column specs.

specs & docs updated too.

feedback welcome.