feat: Add danger codecov plugin

[HazAT]

No description provided.

[getsentry-bot]

	Fails
🚫	Danger failed to run dangerfile.ts.

	Warnings
⚠️	Please add a changelog entry for your changes.

	Messages
📖	✅ TSLint passed
📖	@sentry/browser gzip'ed minified size: 21.9814 kB

Error TypeError

Cannot read property 'toFixed' of undefined
TypeError: Cannot read property 'toFixed' of undefined
    at Object.<anonymous> (/home/travis/build/getsentry/sentry-javascript/node_modules/danger-plugin-codecov/dist/index.js:48:47)
    at Generator.next (<anonymous>)
    at fulfilled (/home/travis/build/getsentry/sentry-javascript/node_modules/danger-plugin-codecov/dist/index.js:4:58)
    at <anonymous>

Dangerfile

43|       });
44|     } else {
45|       message('✅ TSLint passed');
46|     }
47|   });
-------------------------------------------------^
48| 
49|   const hasChangelog = danger.git.modified_files.indexOf('CHANGELOG.md') !== -1;
50|   const isTrivial = (danger.github.pr.body + danger.github.pr.title).includes('#trivial');
51| 

Generated by 🚫 dangerJS

[HazAT]

I will merge this as soon as I debugged my plugin in the evening ^^