inject option function support

[shirotech]

This gives user full control over how the styles are injected. @egoist can you please take a look, would be awesome if this can be merged. Thanks.

[shirotech]

Not sure why circleci can't install node-sass for some reason, tests run fine on my machine.

[cdashrowe]

Not sure why circleci can't install node-sass for some reason, tests run fine on my machine.

Seems to be an issue with the updated version of node circleci is using, having the same issue on my PR.

[codecov]

Codecov Report

Merging #199 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #199      +/-   ##
=========================================
+ Coverage   92.72%   92.8%   +0.07%     
=========================================
  Files           8       8              
  Lines         275     278       +3     
  Branches       99     101       +2     
=========================================
+ Hits          255     258       +3     
  Misses         20      20

Impacted Files	Coverage Δ
src/postcss-loader.js	93.54% <100%> (+0.14%)	⬆️
src/index.js	96.34% <100%> (+0.04%)	⬆️

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[shirotech]

@cdashrowe I've changed node to use lts instead of latest which should be more stable and seems to have fixed the issue.

Would be good if anyone has merging rights to look at this. Many thanks.

[himself65]

landed at 545dc80 and b364cbe