Provide a way to skip some examples

[jason0x43]

Snapguidist should ideally provide a way to filter which examples are tested, or to disable testing for specific code blocks. For example, users may not want snapguidist to run tests on example blocks in general doc sections.

[MicheleBertoli]

+100 @cef62 and I talked a lot about this.
PR anyone? :)

[jason0x43]

One way to go about it would be to give snapguidist's Preview a prop that would disable testing, and have Playground set that on Preview based on a setting passed into Playground, like notest (for consistency with noeditor).

[jason0x43]

That would require duplicating a bit more of styleguidist's Playground's code in snapguidist, but it shouldn't be too bad.