Use same colors as Vim for selection

The Solarized homepage does not appear to indicate what color should be
used for selected text. However the Vim color scheme (which some view
as authoritative) uses base03 text on base01 background for selected
text in visual mode.
Use base00 as selection background

This feels like a reasonable compromise between readability and
contrast.
Merge pull request #29 from jibsen/iss28-selection

Use same colors as Vim for selection