fix(`cli`): print number of warnings

[tommy-mitchell]

Might resolve #166.

Changes the formatter to report the number of warnings and changes the CLI to exit successfully if only warnings are reported:

I want some feedback as well - should printType support printing generic arguments? I.e:

// assert.ts
export const printType = <T>(expression?: any) => {};

// index.test-d.ts
import {printType} from 'tsd';

type Severity = 'error' | 'warning';
printType<Severity>();
//=> 'Type for expression *Severity* is: *'error' | 'warning'*'

My only thought is that printType should only print either the generic or the expression, but not both.

This PR is dependent on #182.

[tommy-mitchell]

I want some feedback as well - should printType support printing generic arguments?

On second thought, I’d rather explore this further (and other changes to assert.ts) in another PR.