Make package more configurable

[nandi95]

I wish all options were available as an argument to the tsd command, as a workaround now I have to have

"tsd": {
    "directory": "tests"
},

in my package json while also calling tds programmatically so i can give it the typingsFile argument:

const diagnostics = await tsd({
    cwd: process.cwd(),
    typingsFile: 'src/index.ts'
});