Begin supporting run

I'd like to be able to just call `run file.py` and have a program DWIM. I'm
working on run as a step in this direction. Define a simple configuration that
maps file extensions to template strings where "$file" is replaced with the
argv[1].

It basically works but there are outstanding TODOs. See the README and source
code for more information.

run/main.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"utils"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		log.Fatal("You can only call run with a single file at a time.")
+	}
+
+	rulesPath := utils.Resolve("run.json", []string{"/home/wpcarro/.config/run/run.json"})
+	b, err := ioutil.ReadFile(rulesPath)
+	if err != nil {
+		log.Fatal("Could not locate a run.json file: ", err)
+	}
+	rules := map[string]string{}
+	err = json.Unmarshal(b, &rules)
+	if err != nil {
+		log.Fatal("Could not decode run.json as JSON: ", err)
+	}
+
+	fileName := os.Args[1]
+	ext := filepath.Ext(fileName)
+	cmd, ok := rules[ext]
+
+	if !ok {
+		log.Fatalf("No rules for extension, %s, have been defined.", ext)
+	}
+
+	// TODO(wpcarro): Support more sophisticated parsing than just string
+	// splitting. To handle 'cases like this'.
+	tokens := strings.Split(strings.Replace(cmd, "$file", fileName, 1), " ")
+	c := exec.Command(tokens[0], tokens[1:]...)
+	err = c.Start()
+	// TODO(wpcarro): Forward STDERR and STDOUT.
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(c.Wait())
+}