Implement basic CLI

This completes the basic API for the command line interface. I went with
getArgs over any of the other third-party libraries due to the simple
nature of the API. No need to complicate and add another dependency.

app/Main.hs

@@ -1,5 +1,35 @@
 module Main where
 
+import System.Environment
+import System.Exit
 import Lib
 
+-- Apply user-supplied arguments from ARGV over parser
 main :: IO ()
+main = getArgs >>= parse
+
+-- Parse input for known commands, flags, or fall through to help
+parse :: [String] -> IO ()
+parse ["add-done"]  = addDone >> exit
+parse ["add-todo"]  = addTodo >> exit
+parse ["standup"]   = standup >> exit
+parse ["-v"]        = version >> exit
+parse ["--version"] = version >> exit
+parse ["-h"]        = help    >> exit
+parse ["--help"]    = help    >> exit
+parse _             = help    >> exit
+
+-- Helper functions for CLI version, usage information, and exit codes
+version, help, exit :: IO ()
+version = putStrLn "v0.0.0"
+help    = putStrLn $
+            unlines [ "standups [COMMAND] [-v | --version] [-h | --help]"
+                    , ""
+                    , "Where COMMAND is one of:"
+                    , ""
+                    , "  new         Reset the in-progress standup"
+                    , "  add-done    Add a task to the in-progress done"
+                    , "  add-todo    Add a task to the in-progress todo"
+                    , "  standups    Run `add-todo` then archive the standup"
+                    ]
+exit    = exitWith ExitSuccess