Add failing test for imported mixin with wrong path

When relative URLs are on, and there's an image referenced via url(...),
if the URL is unquoted, the generated CSS duplicates the base path
Add visitor.js array-based function cache for faster lookups. perf / #1615
Optimize visitor.visitArray() for perf. #1615
Misc. perf optimizations. #1615
New input chunker in parser.js. #1615
Minor fixes. #1615
allow plugins
move the tester into a seperate module so it can be used by other projects
improved alpha channel handling for math ops, removed 'can't substract or divide a color from a number' constraint
Tests and fix for #1689
color-blending-with-transparency: initial implementation
Added rhino header into build.yml. Fixes #1651
Replaced make command for tests running by grunt command.
color-blending-with-transparency: fixed negative result alpha handling (though allowing out-of-range alpha inputs is subject for further discussions/decision)
color-blending-with-transparency: removed debug log statement mistakenly left out
Updated Readme for full examples

Most users and testers would just copy paste the examples. So I added the variables to the small examples itself that it would be testable for anyone.
Fixed line/column numbers in math expr/ops error messages. Updated tests.
Merge pull request #1714 from seven-phases-max/math-op-error-msg-fix

Fix for math expr/ops error messages line/column numbers.
Minifier complains about annotation in non-JSDoc tag

```INFO: [Minify] Minify: Exception in minifier: input:1665: WARNING - Parse error. Non-JSDoc comment has annotations. Did you mean to start it with '/**'?```
Change American spelling to UK spelling