🏗 Add VS Code auto-formatting support for JS and non-JS files

[rsimha]

Background:

We now run two checks on Travis to ensure code correctness / style / formatting:

• gulp lint uses eslint to check JS files
• gulp prettify uses prettier to check non-JS files

Travis builds often fail due to errors from these checks, which can be annoying and waste VM resources.

PR highlights:

• Add VS Code support for auto-formatting JS and non-JS files on save using amphtml's preferred rules
• Add a project specific .vscode/settings.json settings file
• Document the preferred workflow to enable auto-fix-on-save behavior
• Include files in dot directories during lint / prettify

For example, syntax highlighting + auto-correction for an OWNERS file:

[rcebulko]

LGTM

[rsimha]

I've verified this workflow on Linux and Mac OS. All that's left to do is a documentation review by @mrjoro.