decorate - compose decorators for a single prop (mobx4)

[ramybenaroya]

PR checklist:

• Added unit tests
• Updated changelog
• Updated docs (either in the description of this PR as markdown, or as separate PR on the gh-pages branch. Please refer to this PR). For new functionality, at least API.md should be updated
• Added typescript typings
• Verified that there is no significant performance drop (npm run perf)

Support decorate new functionality introduced in #1652 in MobX 4

Changes

• Reduce multiple decorators to a single decorator
• Added a test for multiple decorators (@action + custom) on a function property
• Added a test for multiple decorators (@observable + @serializable) on
  a regular property
• Added a usage example (+ caveat warning) in readme under decorate section

Usage Example:

import { decorate, observable } from "mobx"
import { serializable, primitive } from "serializr"
import persist from 'mobx-persist';

class Todo {
    id = Math.random();
    title = "";
    finished = false;
}
decorate(Todo, {
    title: [serializable(primitive), persist('object'), observable],
    finished: observable
})

[coveralls]

Coverage increased (+0.01%) to 92.801% when pulling a703d0c on ramybenaroya:compose-property-decorator-mobx4 into ad4288d on mobxjs:mobx4-master.

[mweststrate]

Thanks! Will release soon