Export If and Improve docs

[AlfieJones]

🎁 Pull Request

This pr exports 'If' properly and improves it's docs.

After this pr, you can now use

import {A} from 'ts-toolbelt'
type test = A.If<true, string>

• Used a clear / meaningful title for this pull request
• Tested the changes in your own code (on your projects)
• Added / Edited tests to reflect changes (tests folder)
• Have read the Contributing part of the Readme
• Passed npm test

Fixes

Why have you made changes?

What changes have you made?

• changed this to achieve this
• changed that to achieve this
• ...

What tests have you updated?

• tested this in tests/...
• tested that in tests/...
• ...

Is there any breaking changes?

• Yes, I changed the public API & documented it
• Yes, I changed existing tests
• No, I added to the public API & documented it
• No, I added to the existing tests
• I don't know

Anything else worth mentioning?

[sweethuman]

@AlfieJones Want to publish a fork of it with the improvements? Maybe we can merge all the other improvements from other pull requests that haven't yet been merged.