support "env" setting

support selection formatting

Merge pull request mitranim#5 from jostyee/patch-1

Fix a typo

more conventional name for preferences/settings cmd

added command "Fmt: Settings"

opt into Python 3.8 where possible

support env vars; disable fmt-on-save by default

support variable substitution in `cmd`

breaking: proper scope selector matching

Instead of requiring scopes to match exactly, Fmt now allows partial
selector matching. For example, `source.json` will also match
`source.json.sublime`.

Breaking: to accommodate this feature, the setting "scopes" has been
renamed to "rules", and must be a list where each rule has a "selector".
This is more flexible and more consistent with the rest of Sublime.

corrected Go example in settings