feat(serializer): support providing a custom StyleSheet instance

[emmatown]

What:

Let people pass in a custom StyleSheet instance to serializer.

Why:

So that people who use css-in-js solutions like cxs can use jest-glamor-react to test their components.

How:

I wrapped the current serializer in a function that accepts a StyleSheet instance and returns a serializer. To not break the current API I called the function with glamor's styleSheet and put those properties onto the function itself so it can be used in these two ways; first with glamor, second with any other StyleSheet like it.

import { serializer } from 'jest-glamor-react'
expect.addSnapshotSerializer(serializer)

import { serializer } from 'jest-glamor-react'
expect.addSnapshotSerializer(serializer(sheet))

For the tests, I copied the ones from index.test.js and modified them to use cxs. In the README, I used cxs as an example.

Closes #5 Related: tkh44/emotion#21

[codecov[bot]]

Codecov Report

Merging #10 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #10   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          92     95    +3     
  Branches       17     17           
=====================================
+ Hits           92     95    +3

Impacted Files	Coverage Δ
src/serializer.js	100% <100%> (ø)	:arrow_up:

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