Remove `axios.all` and `axios.spread`

Those helpers can easily be replaced with native features (Promise.all and ES6 parameter destructuring) or specialized libraries, so we should restrict our API to HTTP-related methods.

We can deprecate them in 0.x and completely remove then in 1.0.

[qm3ster]

Should the following in the README example:

axios.all([getUserAccount(), getUserPermissions()])
  .then(axios.spread(function (acct, perms) {
    // Both requests are now complete
  }));

be replaced with this instead:

Promise.all([getUserAccount(), getUserPermissions()])
  .then(function ([acct, perms]) {
    // Both requests are now complete
  });

or this:

const [acct, perms] = await Promise.all([getUserAccount(), getUserPermissions()])
// Both requests are now complete

[rubennorte]

Axios uses Promise.all under the hood so you must be polyfilling it already.