feat: add the optional argument cache to the constructor

[tmorin]

Hi,

Could it be possible to provide the instance of the cache to use as an argument of the constructor as below?

const keystore1 = new Keystore()
const keystore2 = new Keystore(undefined, new LRU(10))
const keystore3 = new Keystore(level('./somewhere'), new LRU(10))
const keystore4 = new Keystore(level('./somewhere'))

Or using an options object?

const keystore1 = new Keystore()
const keystore2 = new Keystore({
  cache: new LRU(10)
})
const keystore3 = new Keystore({
  store: level('./somewhere'),
  cache: new LRU(10)
})
const keystore4 = new Keystore({
  store: level('./somewhere')
})
// for compatibilities
const keystore4bis = new Keystore(level('./somewhere'))

If you are agree I can do the pull request with the additional tests.

Best regards

[aphelionz]

What do you think @shamb0t and @haadcode

[shamb0t]

This would be a nice addition, thank you @tmorin! Though it's a breaking change I would prefer the options object approach to avoid passing undefined in the args, what do you think?

[tmorin]

hi @shamb0t , @aphelionz
The options' object is my preference too.
Best regards

[shamb0t]

closed by #42, thanks @tmorin!