Add an option to add a retry delay

[gabaum10]

I'm submitting a...

Feature request

[ ] Regression 
[ ] Bug report
[ x ] Feature request
[ ] Documentation issue or request
[ ] Support request

Current behavior

The retries fire immediately

Expected behavior

An option to configure an optional delay to the retry

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Sometimes the failure is due to a transient network issue that might not be resolved by immediately retrying the asset.

Environment

Assets-retry version: 0.3.4

 
For Tooling issues:
- Browser version: Chrome - Latest
- Platform:  Mac

Others:

[Nikaple]

Scripts must be retried simutaneously, as we use document.write to block html parsing. Consider two scripts vendor.js and main.js, and main.js calls function in vendor.js. If vendor.js failed to load and we retry vendor.js after a certain delay, function calls in main.js would result in an error. See: https://github.com/Nikaple/assets-retry/blob/master/src/util.ts#L164