(feature) (types) Improve brew tap inspector

[skylarmacdonald]

Related to #14 having now been implemented in types/brew-tap.sh

The inspect action on the brew-tap type should:

• Check if the tap comes from anywhere other than GitHub — the type doesn't currently support installing these
• Check if the tap is pinned, and return the correct assertion to replicate this

[skylarmacdonald]

Re #41 — pinning a tap is no longer supported by Homebrew so the type and its inspector do not need to do this. It would be nice to figure out the first part, though (and in turn, figure out how to make the type support non-GitHub taps).