TypeScript declarations

[JakeSidSmith]

Hey, I'd love to use this lib on a new project I'm starting, but I'll be using TypeScript and so will need some types for your lib.

Since I'll be writing them anyway, would you accept a pull request to include the types with your library?

[chromakode]

Sure, that would be great! 👍

[osdiab]

I'm trying react-html-emails out and would be happy to help with this - I see that #48 has been stale for years so assuming this project meets my company's needs, I'll hopefully be adding some soon!

[osdiab]

Oh, just noticed that this library hasn't been updated in 3 years... yeah maybe I won't do it then lol. bummer

[JakeSidSmith]

Unfortunately my project got side-tracked, so I didn't get round to writing these types.