feat: keep_files should be configurable for `bridgetown deploy`

[whysthatso]

Summary

add a keep_files option to the deploy command similar to the existing option in build.

Motivation

the deploy runs Bridgetown::Commands::Clean.start which nukes the destination dir. i use git to deploy the build artifacts to a destination, and would need the .git dir to stick around. the build command has a keep_files option, and i would like to have a similar option for deploy

there's been a brief discussion about a similar issue in #232

[jaredcwhite]

Yeah, this definitely feels like an oversight/bug. Thanks for filing the issue(s).

[whysthatso]

is there a guide how to contribute to the documentation? i could put in a pr possibly.

[jaredcwhite]

@whysthatso So the running of the clean task is just part of the site's customizable Rakefile, so you can definitely just take that out if it's causing an issue. Maybe we can document that option here. I don't think there's a change needed to the internal code.