New option to exit with >0 code if old deps are found

Closes #39

----

BTW, in the README I found that some paragraphs were hard-wrapped and
some were not, so I wasn’t sure what to do. So I did the only logical
thing: introduced an additional wrapping style!

The wrapping style used in the new paragraphs is sometimes called
“semantic linebreaks” and sometimes “one sentence per line”

Some reading on this style, if you’re as geeky as me:

https://rhodesmill.org/brandon/2012/one-sentence-per-line/
https://nathangrigg.com/2012/05/strategic-line-breaks

README.new.md

@@ -93,6 +93,26 @@ Checking virtual versions in: deps.edn
    cider/piggieback {:mvn/version "0.4.1-SNAPSHOT"} -> {:mvn/version "0.4.1-20190222.154954-1"}
 ```
 
+### Non-zero exit code
+
+By default Depot exits with the [exit code][exit-code] `0` (which, by convention, denotes success) whether or not it finds any old/outdated dependencies.
+To have Depot exit with a non-zero exit code when it does find such dependencies, include the `--fail` flag.
+Depot will then set its exit code to the number of old/outdated dependencies.
+
+This can be useful when running Depot as part of a script or in some automated context, such as a [Continuous Delivery][continuous-delivery] pipeline, workflow, or job.
+
+For example:
+
+```bash
+$ clojure -Aoutdated --fail
+Checking for old versions in: deps.edn
+  olical/depot {:mvn/version "..."} -> {:mvn/version "..."}
+  eastwood {:sha "..."} -> {:sha "..."}
+$ echo $?
+2
+```
+
+
 ## Existing work
 
 This project is inspired by [lein-ancient][], it relies on [version-clj][] (by the same author, [xsc][]) for parsing and comparison of version numbers.
@@ -117,6 +137,8 @@ Find the full [unlicense][] in the `UNLICENSE` file, but here's a snippet.
 
 Do what you want. Learn as much as you can. Unlicense more software.
 
+[continuous-delivery]: https://en.wikipedia.org/wiki/Continuous_delivery
+[exit-code]: https://en.wikipedia.org/wiki/Exit_code
 [unlicense]: http://unlicense.org/
 [lein-ancient]: https://github.com/xsc/lein-ancient
 [version-clj]: https://github.com/xsc/version-clj

src/depot/outdated/main.clj

@@ -26,6 +26,9 @@
    ["-e" "--every" "Expand search to all aliases."]
    ["-w" "--write" "Instead of just printing changes, write them back to the file."]
    ["-r" "--resolve-virtual" "Convert -SNAPSHOT/RELEASE/LATEST versions into immutable references."]
+   ;; Not assigning -f to the following option because -f is an extremely common shorthand for a
+   ;; --force option and I don’t want to risk anyone getting confused.
+   [nil  "--fail" "If any old versions are found, exits with a non-zero status code."]
    ["-h" "--help"]])
 
 (def ^:private messages
@@ -37,7 +40,7 @@
                 :no-changes "  All up to date!"}})
 
 (defn -main [& args]
-  (let [{{:keys [aliases consider-types every help write resolve-virtual]} :options
+  (let [{{:keys [aliases consider-types every fail help write resolve-virtual]} :options
          files :arguments
          summary :summary} (cli/parse-opts args cli-options)]
     (cond
@@ -55,10 +58,12 @@
                        (:update-old messages))
             new-versions (if resolve-virtual
                            resolve-virtual/pinned-versions
-                           depot/newer-versions)]
+                           depot/newer-versions)
+            exit-code (if fail (count new-versions) 0)]
         (when (and every aliases)
           (println "--every and --aliases are mutually exclusive.")
           (System/exit 1))
         (run! #(update/apply-new-versions % consider-types check-alias? write messages new-versions)
-              files)))
+              files)
+        (System/exit exit-code)))
     (shutdown-agents)))