Enable -fwarn-incomplete-patterns by default

Motivation

Accidental partial pattern matching is a very common stumbling block for beginners and seasoned users alike. I think enabling the warning -fwarn-incomplete-patterns by default would mean less confusion for everyone.

For example, stackoverflow has over 200 questions from users that are confused by the runtime error: https://stackoverflow.com/search?q=%22Non-exhaustive+patterns%22+%5Bhaskell%5D

Proposal

Add -fwarn-incomplete-patterns to the list of warnings that are enabled by default.