Changlang Kuang

Change-Id: I9cf603df0717a8f174cf71ea6c236967d21d356c Hi, all. This PR make rolling window configurable, and it is backward compatible.

program panic on my first try

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panic: runtime error: index out of range goroutine 1 [running]: panic(0x668ca0, 0xc82000e090) /usr/local/go/src/runtime/panic.go:481 +0x3e6 github.com/natefinch/graffiti/tags.(_visitor).Visit(0xc8200c6930, 0x7faa6a903bc0, 0xc820078f60, 0x0, 0x0) /home/kcln1687/gopath/src/github.com/natefinch/graffiti/tags/generate.go:126 +0x4d6 go/ast.Walk(0x7faa6a903a78, 0xc8200c6930, 0x7faa6a903bc0, 0xc820078f60) /usr/local/go/src/go/ast/walk.go:52 +0x56 go/ast.Walk(0x7faa6a903a78, 0xc8200c6930, 0x7faa6a903ad0,...

fix: Nil method fails on nil pointer

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When I test function like: ```go func foo() map[string]*Foo{ // ... } ``` it returns a map, the value of some keys are nil, and Nil method fails on that...