Allow driver to be set from environment variable

This changeset support for allowing the database driver to
be set from an environment variable. Added a unit test to confirm
behavior.  Resolves #31

db-sample/dbconf.yml

@@ -16,3 +16,7 @@ customimport:
     open: customdriver open
     import: github.com/custom/driver
     dialect: mysql
+
+environment_variable_config:
+    driver: $DB_DRIVER
+    open: $DATABASE_URL

lib/goose/dbconf.go

@@ -41,6 +41,7 @@ func NewDBConf(p, env string, pgschema string) (*DBConf, error) {
 	if err != nil {
 		return nil, err
 	}
+	drv = os.ExpandEnv(drv)
 
 	open, err := f.Get(fmt.Sprintf("%s.open", env))
 	if err != nil {

lib/goose/dbconf_test.go

@@ -1,7 +1,8 @@
 package goose
 
 import (
-	// "fmt"
+	"os"
+	"reflect"
 	"testing"
 )
 
@@ -35,3 +36,35 @@ func TestImportOverride(t *testing.T) {
 		t.Errorf("bad custom import. got %v want %v", got, want)
 	}
 }
+
+func TestDriverSetFromEnvironmentVariable(t *testing.T) {
+
+	databaseUrlEnvVariableKey := "DB_DRIVER"
+	databaseUrlEnvVariableVal := "sqlite3"
+	databaseOpenStringKey := "DATABASE_URL"
+	databaseOpenStringVal := "db.db"
+
+	os.Setenv(databaseUrlEnvVariableKey, databaseUrlEnvVariableVal)
+	os.Setenv(databaseOpenStringKey, databaseOpenStringVal)
+
+	dbconf, err := NewDBConf("../../db-sample", "environment_variable_config", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := reflect.TypeOf(dbconf.Driver.Dialect)
+	want := reflect.TypeOf(&Sqlite3Dialect{})
+
+	if got != want {
+		t.Errorf("Not able to read the driver type from environment variable."+
+			"got %v want %v", got, want)
+	}
+
+	gotOpenString := dbconf.Driver.OpenStr
+	wantOpenString := databaseOpenStringVal
+
+	if gotOpenString != wantOpenString {
+		t.Errorf("Not able to read the open string from the environment."+
+			"got %v want %v", gotOpenString, wantOpenString)
+	}
+}