Consistently use t instead of self
Simplify String()

Since t.Lines() already returns a suitable slice, which we won't mutate,
just the the returned one instead of regenerating one.

This easily avoids generating garbage and reduces memory consumption.
Leverage regexp package for ReplaceAll

In order to replace only once and avoid generating garbage,
leverage Expand functionality of regexp package.

Also adds test for un-colored case.
Use AddStrings to partially implement Add

In order to avoid generating garbage and have a better split of
responsabilities between those function, implement the formatting of any
value in Add and the column width handling in AddStrings.
Merge pull request #3 from nightlyone/simplify_table

Simplify table