add mod file (#14)

go.mod

@@ -0,0 +1,5 @@
+module github.com/valyala/fasttemplate
+
+go 1.12
+
+require github.com/valyala/bytebufferpool v1.0.0

go.sum

@@ -0,0 +1,2 @@
+github.com/valyala/bytebufferpool v1.0.0 h1:GqA5TC/0021Y/b9FG4Oi9Mr3q7XYx6KllzawFIhcdPw=
+github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyCJ6HpOuEn7z0Csc=