added filenames_map.go generation

languagesByFilename now is a map[string]string

internal/code-generator/assets/filenames.go.tmpl

@@ -0,0 +1,11 @@
+package slinguist
+
+// CODE GENERATED AUTOMATICALLY WITH gopkg.in/src-d/simple-linguist.v1/internal/code-generator
+// THIS FILE SHOULD NOT BE EDITED BY HAND
+// Extracted from github/linguist commit: {{ getCommit }}
+
+var languagesByFilename = map[string]string{
+	{{range $filename, $language := . -}}
+	"{{ $filename }}":	{{- printf "%q" $language -}},
+	{{end -}}
+}