Expand ~ and ~user in secrets.providers

[sparr]

Description of changes:

By adding --type path when retrieving the list of providers, provider paths starting with ~ or ~user will undergo expansion. This allows referring to paths relative to the home directory of the current user or a specific user.

My use case for ~ is to put a provider script under my home directory and refer to it from config.providers without having to hard-code my username in my dotfiles.

There is a technically possible but completely implausible regression mode for this PR: Someone has a provider defined starting with "~" which they expect to resolve to a directory with a literal "~" leading its name, located in the cwd(s) from which they call git-secrets. That provider path would fail to resolve after this change.

--path would accomplish the same goal, with more backwards compatibility for older git versions, but less forward compatibility if that deprecated option is ever removed.

Also fixes #159

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sparr]

The old regex was overly broad and should have been at least ^secrets\..* to begin with.
--type path should only be applied to providers, so they have to be broken out here. That means explicitly enumerating the settings keys. Because the tests (and plausible real world use cases) expect --list's exit code to reflect whether any settings were found I had to accumulate a result along the way, and this was the most concise way that I was comfortable with.