Add Displayable Types

[jwest115]

• Define displayable types in FPP spec
• Add isDisplayable to Type trait. Update FPP types to override isDisplayable based on whether the type is displayable or not
• Update Command, Event, TlmChannel, and Param semantics to check whether each contains displayable types (if not, throw and InvalidType error)
• Add unit tests to fpp-check

Closes #236.

[bocchino]

Looks good! I made some revisions to the User's Guide. I added some semantic tests for displayable types, and I added some more error reporting for tracing through type definitions for non-displayable types.