Optionally provide types for input variables

[BLamy]

In #19 we improved Prompt.ts's types to add the ability to infer the keys of the 2nd argument. However, as of right now all values will be typed as a string.

Lets take things a step further and allow users to optionally provide the types for their arguments.

https://www.typescriptlang.org/play?ssl=7&ssc=3&pln=3&pc=1#code/JYWwDg9gTgLgBAbzgZTgXzgMyhEcDkMAzgLQwQQA2ARgKaUz4DcAUCzAJ5i1wCiAHjCgBDAMYwAKrXCVhMWgAVhIkEQA8EhTnCTaguHvkA7ACZE4RIcCMBzADRxN2sDACCUG+YC8fQSPFSMnKKysKqGlq4LlKCDghoAHwJcD4ILHBwANoA0nDWKAB0yGCUwDARzrqxBAgI+AmZRgCuIHRQALpwwua5hrSm5gAGACQI1pi0UHAKaLOjllDWNmiD6RlwAPzTaxkAXHBGtABuk+37vYL9ZnAjY0YTUzNzCAtLK5vTBpcDcADWtBwIJhHJEdO5PB8nFE3B4iJkFJ19q9bHAkVZbKw0KwWJgmkZxMAIEY4KIoLRgqDotISsEKtCYvA+j9kTYEgAKNZgSr7KE6Bl2FgASkQOzgZJgTSgxLS61lcBs-UmwX2GnB5iZ1wEQjEuiC8iUKnUvKp1XiSTZyk8PLVwq8yRlcsd4slxIARFpaMYTHAYAALYDmANdCzomzyxUieTe6gcOAAcQUEldotlaFFabTbE43DgACkIP81SlEHAAFYF2gSLi0fau3FGIwcV1wAA+cFdJha1GbWJYoiJljgXOhxfwUkolDgIB4wkQCHL-yr3FmZYrBWYfYH8AXPB8pPJ+sqgRp8jZw50goKCsOkdoanzhdh7KQO6XNfb9cbPcFLAA9L+4AAOQgAwoBwKY6FEYQmiIHhs1ocwAHdJh4IwIHgEBhGaYQJ1jYcjmAExaG9IgQNfas4EQ4AJzgIjMGghgfRAlk+zJCkj2pWRT3PFxL2vJVTxfCs31rboTEwb82Ckth93Y+lOOCM9Kj4iNaQfWg1WfVdF2rUSiHE78gA