Ignore hidden files as rebuild indicator

Some editors (i.e. emacs) create a new (hidden) file to track changes
to files. Gin picks up on this new .go file and rebuilds. The go tool
chain ignores this file and so should gin.

```bash
$ echo 'package main\nimport "fmt"\nfunc main() {\nfmt.Println("Hello")\n}' | gofmt > .t.go && go build
can't load package: package .: no buildable Go source files in /Users/brian/kill/test
```