feat: accept function as publicPath option

[ipsips]

Allow publicPath option to be a function that returns the publicPath string that could be determined based on current context.

In my use case I need to return a relative path with varying depth depending on how deep in dir structure the loaded file is:

publicPath: (context) => '../'.repeat(path.relative(srcPath, context.context).split('/').length)

[ipsips]

@jhnns what are the chances of this getting a review/merge/publish to npm any time soon? jus' hate to depend on my fork in our project :)

[jhnns]

Thank you for the PR. Looks good 👍

Could you also add a small explanation to the README?

[ipsips]

certainly. added an explanation and example to Options section in README

[jhnns]

Thx 👍. Published as 3.1.0 🚀