feat: allow to include html partials using <webpack-include src="./partial.html" />

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

simple template

Breaking Changes

No

Additional Info

In future we will support <webpack-partial>Foo Bar</webpack-partial>, it will be like <template> and <slot>

[codecov]

Codecov Report

Merging #364 (6d4ac58) into master (6654c9d) will decrease coverage by 10.77%.
The diff coverage is 91.30%.

@@             Coverage Diff             @@
##           master     #364       +/-   ##
===========================================
- Coverage   97.71%   86.94%   -10.78%     
===========================================
  Files           7        7               
  Lines         526      544       +18     
  Branches      143      148        +5     
===========================================
- Hits          514      473       -41     
- Misses         12       67       +55     
- Partials        0        4        +4     

Impacted Files	Coverage Δ
src/utils.js	91.06% <90.00%> (-6.87%)	⬇️
src/index.js	100.00% <100.00%> (ø)
src/plugins/sources-plugin.js	92.95% <100.00%> (-4.19%)	⬇️
src/HtmlSourceError.js	0.00% <0.00%> (-93.34%)	⬇️

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