Include a launchd .plist file.

macOS has two ways of running scheduled tasks:
1. Traditional Unix cron
2. launchd

According to https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/ScheduledJobs.html, Apple strongly prefers the launchd model. This commit introduces the com.stevegrunwell.asimov.plist file, which can be loaded into the launchd system via `launchctl load ./com.stevegrunwell.asimov.plist`.

It's not perfect, namely because it relies on the `asimov` executable being available at /usr/local/bin/asimov; ideally, users should be able to put this anywhere, but it seems that launchd is pretty picky about paths. Fortunately, this will become less of an issue once Asimov is available via Homebrew (#2).

com.stevegrunwell.asimov.plist

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+    <dict>
+        <key>Label</key>
+        <string>com.stevegrunwell.asimov</string>
+        <key>Program</key>
+        <string>/usr/local/bin/asimov</string>
+        <key>StartInterval</key>
+        <!-- 24 hours = 60 * 60 * 24 -->
+        <integer>86400</integer>
+    </dict>
+</plist>