Allow creating new assertions

[rstacruz]

Just a proof of concept of #32.

[rstacruz]

so whats the verdict? :-)

[mjackson]

I think the only thing that makes me a little uncomfortable about this API is that it's not explicit enough.

In jQuery, you have this global jQuery object and if you want to use a plugin you just drop another <script> tag in your page. If we use that pattern, people will end up doing something like:

import expect from 'expect'
import extension from 'expect-extension'

// they won't actually use the "extension" var down here, so linters
// will complain about an unused var

What if we did this instead?

import expect from 'expect'
import extension from 'expect-extension'

// here we actually use the "extension" var
expect.extend(extension)

Also, this way extension authors won't need to put anything on expect.fn. Instead, they can just:

import { assert } from 'expect'

export function toBeAColor(value) {
  assert(
    isColor(value),
    'Expected %s to be a color, but it is not',
    value
  )
}

[rstacruz]

oh, perfect.

[rstacruz]

Actually, we can go one step further and export assert as this.assert. This way, an expect extension doesn't need to have a direct dependency on expect. What do you think?