Add launchd daemon support for macos (#16)

* Add command to install LaunchAgent

* Move `rdm service` to `rdm service install` subcommand

* wip

* state to new package

* Clean up redundant code

* More tweaks

* Poll for service to come up

* Move launchd code into a new library

https://github.com/brasic/launchd

Also add support for starting, stopping and uninstalling the service.

* Remove a way things can fail

This pulls in a breaking change from launchd:
brasic/launchd@f5c0c73

Now the signature of ForRunningProgram has no error so we don't need to
handle it.

* Update README.md

Co-authored-by: Blake Williams <blakewilliams@github.com>

* Update internal/cmd/service.go

* Gracefully shutdown the http server

Some logging changes exposed the fact that we were not actually shutting
down the server on SIGINT/TERM. Change that, and add some entries to the
server log indicating when a server came up and went down.

* Update brasic/launchd to v1.0.1 to pull bugfix

brasic/launchd@5335064

* Update to universal logger.

This consolidates everything under a single logger which is unprefixed
and delivered to stderr when being used for human-readable messages (ie
CLI context) and timestamp-prefixed and multiplexed to both the log file and
stdout when used in server context. We manage the transition from CLI
to long-lived process by reconfiguring the existing logger built in
main.go. One advantage of this is that things that need to log inside
main.go but the server log should know about (like signal receipt) get
sent to the right place instead of a now-likely-disconnected stderr.

* Fix test interdependence

These tests are order dependendent and not parallel safe, apparently because
some of them launch goroutines that outlive the tests themselves yet
continue to use package global resources like httpClient and the
implicitly global unix socket at path. Give each test its own copy of
these resources to fix flaky tests.

---------

Co-authored-by: Blake Williams <blakewilliams@github.com>

Properly support reading stdin (#8)

Currently we read from stdin and always append a newline due to
`scanner.Scan` splitting on newlines. This isn't always correct, since
input doesn't always have a newline.

This resolves the issue by using `bufio.Reader.ReadBytes` which reads
up-to the delimiter and includes it, preserving newlines.

Update README.md (#4)

Updates the code snippet to point to a valid file

Add more info to README

Remove unused code, fix ssh path

Revert "Support older Go's"

This reverts commit 70e641f.