Read the color pairs in top to bottom order, instead of prior complicated manner

This makes it so both g:rbt_colorpairs in the user .vimrc and s:pairs in
the autoload are easier to understand, with the highest level scope
being the first entry, and so on

The behavior this replaces is to roll back from the end of the list, with odd
truncation stuff happening with the max trimming, making it completely
unpredictable what color will be where for certain size lists

This patch makes it so the first item in the list (['brown',
'RoyalBlue3'] in pairs) is the top level scope in the file, and so on.