Add a flag or command to display a list of user-configured plugins

[CruzMolina]

Directly related to trufflesuite/trufflesuite.com#1396.

Would be nice to get a list of properly configured/installed plugins, perhaps a --list flag to call with truffle run (truffle run --list). A truffle list command might be overkill and hinges on how much we expand plugin capabilities (perhaps worth pursuing later on). Might also be cool to expand this to check for all global or local cwd plugins (truffle-plugin.json's) regardless of whether a user has them configured correctly or not.

(Self-assigning to keep open, but anyone free to work on it!)