Make scopeSelector a trailing option to Config::set
Make scope a trailing option to Config::get
Use the user’s config.cson path as the default config source
Pass source to ::scopedSettingsStore on calls to ::set

Default it to the user’s config if no source is specified.
Add a ‘sources’ and ‘excludeSources’ options to Config::get

If the option is supplied, we will only retrieve values from the
specified sources.
Don’t return schema defaults when sources are specified in Config::get
Use ‘source’ option rather than internal state in Config spec
Allow null keyPath with options argument in ::get
Use a schema in all specs for Config::observeUserConfig

Before, values loaded from the config file were set in a
way that allowed any key to be set in the root object.
Now, these values are set in a way that's similar to how
::set works.
Fix deprecation in package-manager-spec
Take 'scope' option in Config::observe

Deprecate using the scope as an optional first argument
Take 'scope' option in Config::onDidChange

Deprecate passing the scope as an optional first argument
Throw when Config::set is called w/ source and no scope
Add Config::unset

Deprecate Config::restoreDefault
Fix config priorities
:memo: new config APIs
Deprecate Config::isDefault and ::getDefault
Flesh out deprecation messages for Config::{is,set}Default

Also, simplify the shim in ::isDefault
Remove toBe parens
:lipstick: