Handle views in combo with Ace base templates

As views looks like a regular template, but doesn't need a base template, we have to look inside it.

Altough really not needed by this commit, reading the full file content into memory just to do a substring search is a waste.
So this commit implements a `ReaderContains` func that in most cases should be much faster than doing an `ioutil.ReadAll` and `bytes.Contains`:

```
benchmark                   old ns/op     new ns/op     delta
BenchmarkReaderContains     78452         20260         -74.18%

benchmark                   old allocs     new allocs     delta
BenchmarkReaderContains     46             20             -56.52%

benchmark                   old bytes     new bytes     delta
BenchmarkReaderContains     46496         1258          -97.29%
```

Fixes #999

helpers/path.go

@@ -16,16 +16,15 @@ package helpers
 import (
 	"errors"
 	"fmt"
+	"github.com/spf13/afero"
+	jww "github.com/spf13/jwalterweatherman"
+	"github.com/spf13/viper"
 	"io"
 	"os"
 	"path/filepath"
 	"regexp"
 	"strings"
 	"unicode"
-
-	"github.com/spf13/afero"
-	jww "github.com/spf13/jwalterweatherman"
-	"github.com/spf13/viper"
 )
 
 // FilepathPathBridge is a bridge for common functionality in filepath vs path
@@ -153,6 +152,17 @@ func IsEmpty(path string, fs afero.Fs) (bool, error) {
 	return fi.Size() == 0, nil
 }
 
+// Check if a file contains a specified string.
+func FileContains(filename string, subslice []byte, fs afero.Fs) (bool, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return false, err
+	}
+	defer f.Close()
+
+	return ReaderContains(f, subslice), nil
+}
+
 // Check if a file or directory exists.
 func Exists(path string, fs afero.Fs) (bool, error) {
 	_, err := fs.Stat(path)