Fix global Cypress namespace augumentation

[miluoshi]

This is fixed with 2 things:

1. For global namespace augumentation to work correctly, the augumentation needs to be in an "ambient" context (vs module context).
   You can achieve ambient context in d.ts files in 2 ways:
   a.) by not using import/export keywords (using them toggles module context)
   b.) by wrapping namespace augumentation in declare global {} block - ambient context is inside this block.

See more info here:

• https://www.typescriptlang.org/docs/handbook/release-notes/typescript-1-8.html#augmenting-globalmodule-scope-from-modules
• Clarify what "ambient" means microsoft/TypeScript-Handbook#180 (comment)

2. For Typescript to be able to use provided type declarations, we need to point it to a correct d.ts file.
   This is done by specifying path to this file in package.json's field "types".
   With this, an application just needs to import 'cypress-mailosaur' and types just work

fixes #12

[adjerbetian]

Just a suggestion en passant (I know it was like this before the PR):

I've usually seen a package's definitions in a index.d.ts at the package's root folder, next to the index.js. It won't change anything for a consumer point of view, but it feels weird to have the definitions inside src/ where it's actually not used.