feature: add support for additional cache keys option

[Rohit-L]

PR for issue #303

This adds an option for user to pass additional keys to the default cache key generator for more customization, allowing the user to have more control, if needed, over what hits/misses in the cache.

[jsf-clabot]

All committers have signed the CLA.

[alexander-akait]

Two small fix 👍 Very good works! Thanks!

[alexander-akait]

Bad example, many people can use this from README, just use myCustomVariable name or something like

[alexander-akait]

Please also use other variable

[Rohit-L]

👍 Updated with suggestions from @evilebottnawi

[ooflorent]

LGTM. Not a fan of the complex default keys documentation. I think the README does not require so many details about the plugin internals.

[alexander-akait]

@ooflorent i am not fan also, but it should be documented because in some case it is only one variant to invalidate cache

[shellscape]

In the past I've taken large, complex portions of a README and moved them out into separate .md files in a ./docs directory. That might be considered to keep the README a little smaller. I would like to review this PR but I know practically nothing about how the plugin works, so I'm afraid that my review wouldn't be very useful.

[alexander-akait]

@shellscape i think we can do this in other PR, not here

[Rohit-L]

@evilebottnawi @ooflorent what do you think about making additionalCacheKeys a function that returns the object of additional keys instead?

I just realized, for my use-case, I need access to file when defining my additional cache keys and this implementation wouldn't allow that.

I'm proposing something similar to how cache-loader defines their cacheKey option.

[alexander-akait]

@Rohit-L can you provide example? Go ahead 👍 We need this option to implement other feature - custom minify function for support uglify-js and terser

[alexander-akait]

/cc @Rohit-L friendly ping

[alexander-akait]

/cc @Rohit-L universal solution #320

[alexander-akait]

Close in favor #320