Support native Date type, Components, improved RegExp, new CustomValue API

[Akryum]

Fix #475 Fix #460

This PR improves Date and components support:

• Dates are now treated as native (like RegExp) and are fomated as such.
• Dates and reg exps are now correctly revived when time traveling in vuex.
• Import/Export of dates and reg exps now works too.

• New _custom API.

Example for components:

{
    _custom: {
      type: 'component',
      id: instance.__VUE_DEVTOOLS_UID__,
      display: getInstanceName(instance),
      value: { /* component state */ },
      fields: {
        abstract: true
      }
    }
  }

Other example: https://gist.github.com/Akryum/f2e08df47088d5be0f933b67e90261dc

• Components are now handled in a specific way using above new feature:

[Akryum]

@yyx990803 This is to prevent the default replacer to turn Dates into Strings before calling the custom replacer. See WebReflection/circular-json#4

[Akryum]

I added a new _custom object API to be able to send complex customizable data to the devtools state inspector. This currently allows displaying components in a specific way. See OP.

[Akryum]

I have made some changes on #486 to make the CustomValue API much more powerful. For example: https://gist.github.com/Akryum/f2e08df47088d5be0f933b67e90261dc

[posva]

minor, but you can write this as if (!list.length) return

[posva]

Looks good to me, tested too 🙂
I think some of the code in the devtools could use some comments as my personal experience with it is always a bit hard: it's not easy to get into it to do a small feature. I wonder if you felt that way too at the beginning @Akryum