Add baseUrl option, rename prefixUrl, and allow leading slashes in input

[sholladay]

This PR aims to improve the flexibility of input URLs and options for resolving them prior to the request.

• prefixUrl is renamed to startPath
  • As before, when this option is used, it is prepended to input before any other processing takes place
  • startPath can still be a full URL if needed, however the name is meant to signal its primary use case is a simple base path without a domain, e.g. /api
• A new baseUrl option is added
  • When this option is used, input (including any startPath) is resolved against the baseUrl according to standard URL resolution rules
  • baseUrl will respect the HTML <base> tag if present, and is similar to it, except that it only applies to Ky
• input can now be any valid URL, with or without a leading slash
  • Slashes are automatically normalized when using startPath
  • Slashes are kept as-is and respected in accordance with standard URL resolution rules when using baseUrl

ky('books', { startPath : '/api' });  // GET /api/books
ky('books', { baseUrl : '/api' });  // GET /books
ky('books', { baseUrl : '/api/' });  // GET /api/books
ky('./books', { baseUrl : '/api/' });  // GET /api/books
ky('/books', { baseUrl : '/api/' });  // GET /books
ky('http://bar.com', { startPath : 'http://foo.com' });  // GET http://foo.com/http://bar.com
ky('http://bar.com', { baseUrl : 'http://foo.com' });  // GET http://bar.com
ky('books', { baseUrl : 'http://foo.com', startPath : '/api' });  // GET http://foo.com/api/books

Possible names for the string that is prepended to input

• startPath (as is currently implemented here)
• pathPrefix
• prefixPath
• rootPath

TODO:

• [ ] Finalize name for startPath option
• [ ] Documentation
• [ ] TypeScript types