feat: add option inject

[skimi]

This adds an inject option to conditionally inject into the HTML.
This is useful when you generate multiple html files and want to inject some files only into some of those html files. Also for ENV based build and maybe more.

This is inspired by the favicons plugin https://github.com/jantimon/favicons-webpack-plugin/blob/master/src/index.js#L82 which has the same option that I'm already using for a project.

[codecov-io]

Codecov Report

Merging #174 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #174   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           56        59    +3     
  Branches        13        15    +2     
=========================================
+ Hits            56        59    +3     

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)

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