message: add border examples to Block
diff: import React from 'react'
  -import {colors} from '../../src/theme'
  +import theme from '../../src/theme'
  import {Block, Text} from '../../src'
  -const backgroundColors = Object.keys(colors.bg)
  - .filter(color => color !== 'orange')
  -
  +const colors = Object.keys(theme.colors.bg)
  const textColors = ['white', 'gray', 'black']
  const BlockExample = {
  @@ -12,19 +10,39 @@ const BlockExample = {
  element: (
  <Block p={4}>
  <table>
  + <thead>
  + <tr>
  + <th className="text-left">
  + <Text tag='div' pb={4} mono>color</Text>
  + </th>
  + <th colspan={textColors.length}>
  + <Text tag='div' pb={4} mono>bg={`{color}`}</Text>
  + </th>
  + <th>
  + <Text tag='div' pb={4} mono>borderColor</Text>
  + </th>
  + </tr>
  + </thead>
  <tbody>
  - {backgroundColors.map((bg, i) => (
  + {colors.map((color, i) => (
  <tr key={i}>
  <td>
  - <Text mono nowrap>{`bg='${bg}'`}</Text>
  + <Text mono nowrap mr={3}>{color}</Text>
  </td>
  {textColors.map((fg, j) => (
  <td key={j}>
  - <Block p={3} mb={2} bg={bg} border={bg === 'white'}>
  - <Text color={fg}>{fg}</Text>
  + <Block p={3} mb={2} bg={color} fg={fg} border={color === 'white'}>
  + <Text mono>{fg}</Text>
  </Block>
  </td>
  ))}
  + <td>
  + {(color in theme.colors.border) ? (
  + <Block p={3} mb={2} ml={3} borderColor={color}>
  + <Text mono>{color}</Text>
  + </Block>
  + ) : null}
  + </td>
  </tr>
  ))}
  </tbody>
  