Merge rrweb, rrweb-snapshot, rrweb-player into a mono repository

[YunFeng0817]

1. In this PR, I use yarn workspaces to link multiple packages.
2. Update Travis-CI config.

Related issue #582

[YunFeng0817]

This PR is almost done except for a build warning.
Tips:

1. Because we want to keep the commit history of the other two projects, the number of commits in this PR may be a little too much. And because of the same reason, when merging this PR, we should avoid the squash operation.
2. I think we can keep using yarn workspace as our package manager for now and it's not hard to replace it with another manager if yarn can't meet our requirement later.
3. There's still a build warning as follows in the rrweb-player package.

(!) Unresolved dependencies
[https://rollupjs.org/guide/en/#warning-treating-module-as-external-dependency](https://rollupjs.org/guide/en/#warning-treating-module-as-external-dependency)
rrweb (imported by src/Player.svelte, src/Controller.svelte)
(!) Missing global variable name
Use output.globals to specify browser global variable names corresponding to external modules
rrweb (guessing 'rrweb')
created public/bundle.js in 1.8s

It seems like its rollup config doesn't fit for monorepo environment and any help would be great.

[Juice10]

Looking really good @Mark-Fenng! I found some very minor outdated urls, but apart from that this looks good!

[Yuyz0112]

Nice! Let's go.

I think contributors may see some conflicts in their existing PRs, we can help them get things done again.

And I'm going to check the rollup things and update the release script.

Awesome work.