Add prebuild script and tsconfig to generate declarations

[nathanhleung]

Resolves #41

Relevant issues:

• Allow emitDeclaration and isolatedModules together microsoft/TypeScript#29490 (emitDeclaration is not compatible with isolatedModules, which is already set in the base tsconfig.json, so an alternate config was needed)
• https://github.com/rollup/rollup-plugin-typescript/issues/54 (rollup-plugin-typescript doesn't support generating declarations, so we have to use tsc itself to generate the declarations)

Caveats:

• Not really loving the use of ! and any, but this seems to be something that can only be resolved with 1. some refactoring of types in the upstream shepherd library or 2. some sort of explicit string check (e.g. if (type === 'next') action = tour.next) which means that every time you add a new action type to the base library, you need to add another check here, which I think goes against the spirit of this being a very lightweight, mostly independent-of-upstream wrapper, but let me know your thoughts

Alternative solutions:

• rollup-plugin-typescript2 seems to support generating declarations, so we could also try upgrading

[RobbieTheWagner]

@nathanhleung this seems fine to me. I'm also fine with updating and generating declarations, as you mentioned. Just let us know your preference!

[nathanhleung]

@rwwagner90 If this is fine with you it's fine with me! Feel free to merge in whenever and looking forward to the next release.

[RobbieTheWagner]

@chuckcarpenter I defer to you on this. Feel free to merge, if you think it's good to go.

[chuckcarpenter]

LGTM! Thank you!

[nathanhleung]

@chuckcarpenter Awesome, thanks for LGTMing and hope you had a great thanksgiving holiday! Any idea when you'll cut the next release?