Add custom linter for return types

[This builtin rule](https://palantir.github.io/tslint/rules/typedef/) will check that there's a declared return type for functions, which I think is overkill. But whenever a function has multiple `return` statements (or multiple `yield` statements in a generator), return type should be required.

Possible option: only require a type declaration if the return statements have different types.

This will require a custom linter.

This issue is being filed because it was too complex to fit into https://github.com/css-blocks/css-blocks/pull/65

Start by reviewing the linked TSLint typedef rule and the context in css-blocks#65, since no repository file or test is named here. Define the intended behavior for functions and generators with multiple return or yield statements, including whether differing inferred types change the requirement, then add tests showing the chosen rule is enforced.

Written by the indexing model from the issue text.