Add a toml linter (#74)

* Add a toml linter

* Use if/else instead of os.Exit(0)

* Add usage warning about destructive changes

cmd/tomll/main.go

diff --git a/cmd/tomll/main.go b/cmd/tomll/main.go
new file mode 100644
index 0000000..f185c56
--- /dev/null
+++ b/cmd/tomll/main.go
@@ -0,0 +1,61 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+
+	"github.com/pelletier/go-toml"
+)
+
+func main() {
+	flag.Usage = func() {
+		fmt.Fprintln(os.Stderr, `tomll can be used in two ways:
+Writing to STDIN and reading from STDOUT:
+  cat file.toml | tomll > file.toml
+
+Reading and updating a list of files:
+  tomll a.toml b.toml c.toml
+
+When given a list of files, tomll will modify all files in place without asking.
+`)
+	}
+	flag.Parse()
+	// read from stdin and print to stdout
+	if flag.NArg() == 0 {
+		s, err := lintReader(os.Stdin)
+		if err != nil {
+			io.WriteString(os.Stderr, err.Error())
+			os.Exit(-1)
+		}
+		io.WriteString(os.Stdout, s)
+	} else {
+		// otherwise modify a list of files
+		for _, filename := range flag.Args() {
+			s, err := lintFile(filename)
+			if err != nil {
+				io.WriteString(os.Stderr, err.Error())
+				os.Exit(-1)
+			}
+			ioutil.WriteFile(filename, []byte(s), 0644)
+		}
+	}
+}
+
+func lintFile(filename string) (string, error) {
+	tree, err := toml.LoadFile(filename)
+	if err != nil {
+		return "", err
+	}
+	return tree.String(), nil
+}
+
+func lintReader(r io.Reader) (string, error) {
+	tree, err := toml.LoadReader(r)
+	if err != nil {
+		return "", err
+	}
+	return tree.String(), nil
+}