DesignTokenDocBlock Tab per Context

[jbltx]

Joining #14 and #15, it would be awesome to support multiple contexts (more generic than "themes") and organize tokens per context in tabs.

Like this example:

<DesignTokenDocTabs>
  <DesignTokenDocBlock categoryName="Color Base Dark" tabName="Dark" />
  <DesignTokenDocBlock categoryName="Color Base Light" tabName="Light" />
</DesignTokenDocTabs>

// or 

<DesignTokenDocTabs categoryNames="Color Base *" />

[Sqrrl]

I like your first design, which should work without any breaking changes. Just don't know when I will be able to give it a go. A PR would be really appreciated.