Add a structured log system

In an effort to support future UI work in app.netlify.com, we need to allow for plugins to output logs.

These logs will:

• Be consumed by netlify UI
• Be shippable to third party logging systems
• …

The idea is to capture console.log and any other output returned from plugins and save them to a structured log file.

Those structured log objects should:

• be converted to human-friendly console output. I.e. the user does not see them, they are under the hood.
• be output to files
• be passed to plugins such as log shipping plugins

[mraerino]

Please see the internal issue that david linked for more.

[jlengstorf]

@RaeesBhatti @ehmicky monkey-patching or capturing are tricky — providing an explicit output API and ignoring console logs is what I’d love to see instead

there’s no way to be sure whether another library is monkey-patching console, and if someone installs a noisy package we don’t necessarily want that in the log output

people are already learning how to create build plugins, so telling them that “the way to print a message in Netlify’s build log is to import the logger util” doesn’t seem unreasonable or overly cumbersome