feat(useable): add insertInto support (options.insertInto)

[jasonenglish]

What kind of change does this PR introduce?

feature. adds support for insertInto with useable. fixes #327

Did you add tests for your changes?
Yes

If relevant, did you update the README?
I didn't, I figured this feature should be assumed to work with useable as well. I can add documentation to point it out though.

Summary

This feature is needed for injecting styles into the iframe with useable AFTER the page has loaded. Having the ability to call style.use() when the iframe is being setup is crucial. In this scenario we can't use the basic style-loader because specifying the insertInto option will attempt to look for the iframe when it doesn't exist yet.

Does this PR introduce a breaking change?

No

Other information
n/a

[codecov]

Codecov Report

Merging #341 into master will decrease coverage by 1.33%.
The diff coverage is 80%.

@@            Coverage Diff            @@
##           master    #341      +/-   ##
=========================================
- Coverage   98.43%   97.1%   -1.34%     
=========================================
  Files           4       4              
  Lines          64      69       +5     
  Branches       21      23       +2     
=========================================
+ Hits           63      67       +4     
- Misses          1       2       +1

Impacted Files	Coverage Δ
useable.js	94.11% <80%> (-5.89%)	⬇️

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[michael-ciniawsky]

@jasonenglish Thx

[alexander-akait]

/cc @michael-ciniawsky need release 👍

[michael-ciniawsky]

Released in v0.23.0 🎉