Add typedef rule

[sindresorhus]

https://palantir.github.io/tslint/rules/typedef/

You can see examples of the different options here: https://github.com/palantir/tslint/tree/c01bb2cdc258da4f9727781151d91773fba3e283/test/rules/typedef

[sindresorhus]

@SamVerschueren What do you think about this rule? I kinda like having to be explicit about types, but I noticed it caused as lot of violations in Ow.

[SamVerschueren]

I'm +1 on this. I sometimes write a complex function but end up returning different types which would've been caught by this.

I noticed it caused as lot of violations in Ow.

We should fix it :)

[sindresorhus]

I dropped variable-declaration. It's just too annoying. It requires changing const x = 'foo'; to const x: string = 'foo';, which is totally meaningless.