Add env to options in index.d.ts

[filipbudisa]

Fixes #117

This PR introduces 4 changes to the exec function typescript definition, making it compliant with the javascript code.

• Makes parameters options and callback optional
• If only 2 parameters are passed to exec, the second parameter is the callback function
• Adds the optional env field to options
• Allows callback stdout and stderr fields to be Buffers

[filipbudisa]

0ae7ed8 allows exec to be called with only 1 or 2 parameters, allowing the second parameter to be the callback. Also fixed callback parameter types.

[filipbudisa]

@jorangreef any chance of this getting merged?