Fixed s:GetCommentEnd(), s:EndCommentExpr()

Also fixed s:ConvertDelimiterToSeparator() such that it doesn't mess up
the end comment and draws the border before it.
Fixed #8

- Renamed option for eval_expr.
- Defined a new option `g:table_mode_map_prefix` set to `<Leader>t` by
  default to be used as a prefix for all other table mode commands.
- Updated docs to reflect the same.
- Updated README.